TLDR Version: While Space Timmy himself might not be necessary, the role that he fills most certainly is. To find out why, you're just gonna have to read the long version, or at least the last few paragraphs. P.S. If you don't like to read, I suggest getting off the internet...as it requires quite a lot of reading. 
Alright, we all know that one of the biggest rage-inducing factors of the ME 3 ending was lil' Space Timmy. People thought he was shoe-horned in, that he utterly broke the flow of the game, made certain elements of the game meaningless, etc etc etc. I forget what topic it was in, but I had a discussion with someone regarding this matter and the more I discussed it, the more I came to realize that while you might not like Space Timmy himself, SOMEONE had to fill the role that he filled.
As for complaints about him, I might as well start with this list as provided in a different topic by I.Muir
Timmy specifically describes himself as the collective will of the Reapers. It could be argued that unless the Reapers are awake and active in the galaxy, Timmy doesn't exist. That is to say maybe when it's said "Sovereign must go activate the Citadel relay", what he's really doing is waking up Timmy who in turn activates the relay. While ordinarily this can be done remotely, the Protheans altered the signals that are being sent and received by the Citadel, thus making it impossible for Timmy to be awoken. They accomplished this by returning to the Citadel only once the Reapers had left the galaxy and thus Timmy would be "asleep" again.
If that doesn't work for you, who's to say that Timmy has any control over the Citadel at all? Where did that theory come from? All we know is that he is an AI that is housed inside the Citadel, it's never said that he has full control over it's functionality.
2: (And this ties in with the "shoe-horned in" complaint) The Reapers have always had motivation, they can't stop talking about it. "We represent order, you represent chaos. We bring order to the chaos of organic life. Your mind cannot possibly comprehend the magnitude of our purpose. Yadda Yadda Yadda". Furthermore, if you were paying attention to the conversation with the Prothean VI on Thessia, you'd recall the VI stating that the Reapers are a symptom (or something) of the cycle, however they are not the controlers of it. They too are just pawns. "So who's really in charge"? We don't know, "It's presence is inferred rather than observed." Right there he tells you that there's something bigger than the Reapers that's controlling them. It's what we in the writers' community like to call foreshadowing. That right there, beyond anything else, refutes the "shoe-horned in" complaint as they tell you about him earlier in the game.
As for the motivation, like I said, you couldn't keep them from talking about it. "We are your salvation through destruction" - Harbinger, ME 2 gun-fight taunt. Well it seems the Reapers were certainly right about one thing: we cannot possibly comprehend their motivations. That...that actually directly answers the "Yo Dawg!" motivational poster pic saying "I heard you didn't want to get killed by synthetics so I'm going to kill you with synthetics!" Again, for those in the class that were paying attention, this is something that was addressed in ME 2, and even in your conversation with Soveriegn back in the first game. Surely Harbinger HAD to mean something when he said lines like "We are your salvation through destruction". But this is a fact that apparently very many people either didn't or couldn't understand. The reason "I'm gonna use synths to kill you so you don't get killed by synths" makes no sense is because it's not supposed to...hence "You cannot comprehend our motivations". The Reapers had been preaching it all along.
3: As for how each ending ACTUALLY plays out, here are my thoughts on the matter:
Green: Shepard's DNA is converted into energy and mixes with the Catalyst's (Timmy, who is an AI) ennergy and this combination is sent out in a shockwave that encompasses the entire galaxy. Synthetics become more organic and organics become more synthetis, thus creating a nice (boring) utopia.
Blue: Everything that Shepard is, his/her will, memories, thoughts, everything, is absorbed into the Catalyst, thus making Shepard the new Catalyst. If you notice in the EC, the Citadel doesn't actually fire a beam for this ending, rather it just sends out a shockwave. This shockwave is essentially a "softwave update" that informs all Reapers that Shepard is now in control.
Red: Destroying the power center sends out what is essentially a hyper EMP that fries all Reapers in the galaxy, as well as - like Timmy says - "damaging technology that you depend on, but it should not take long to rebuild".
In short: Space Magic. I don't see how this is a problem my friends, I've heard plenty of people throw around the phrase "Space Magic" with regards to how the endings are bullshit...well in that case the entire series is bullshit as it revolves around space magic that allows people to travel faster than light, move things with their mind, and fire guns that shoot at relativistic speeds without their arms getting ripped off.
4: With regards to the Illusion of Choice. This doesn't necessarily have to do with Space Timmy, but rather the fact that many people would have prefered having your ending being dictated by the choices you made rather than you being able to choose the ending. When you stop and think about all the major choices that you make throughout the game...what is their end purpose? What is their overall goal? When you stop and think about it, with the exception of some "spur of the moment decisions" like killing Wrex on Virmire or destroying/keeping the Collector base, each choice is ultimately building towards the united galactic fleet and/or building the crucible. That's where you get to see your choices played out. Make good choices and your fleet is huge, make back choices and your fleet is small. Killing the rachni means they're not there to help build the crucible. Saving the geth mean they're available to join your fleet and help build the crucible. Betraying the Krogan mean the Salarians join up, saving the Krogan mean they don't. All the choices in one way or another affect the size of your united galactic fleet. While I do agree it would have been nice if said fleet had a bigger impact than just a few cutscenes (I had heard something about them originally wanting there to be a tactical section where you had to issue orders to the fleet and ground troops, kinda like picking teams at the end of ME 2), in the end your choices are building towards either helping or hindering the fleet.
5: This one actually segues perfectly into where I was going to go with this after having addressed the rest of the list: "Why Space Timmy?"
Which brings me to the thesis of my defense, if it wasn't Space Timmy, SOMEONE had to be there to guide Shepard. Beyond the fact that, as I had already mentioned, the Prothean VI foreshadows the fact that there's something controling the Reapers and the Cycle, there had to be something other than Harbinger sitting there at the end of the story. "Why the hell is that? I wouldn't have minded having a conversation with Harbinger and making a choice, why the fuck is it some ghost kid?" Simple. Would you REALLY have been satisfied with a conversation with Harbinger explaining the various uses of the Crucible? Harbinger, Mr. "OMG LULZ ORGANICS ARE WORTHLESS, INSIGNIFIGANT ANTS! THEY THINK THEY CAN WIN WHEN I HAZ CHEEZBURGER?!" Well, you get the point. He goes from gloom and doom, you're wasting your time, your struggle is futile, to all of a sudden saying "And if you choose THIS option you can REALLY fuck us in the ass by enslaving us and making us obey your very will. Oh all that talk about you being worthless? Well I honestly didn't think you'd get this far..." It would be a HUGE character shift for Harbinger to go from explaining how everything you do is meaningless to explaining the various ways you can utterly defeat the Reapers via destruction, control, or pacification.
"Hey douchefag, you STILL have said why the role is necessary." Hold your nuts, I'm getting to that. For starters - and a rather simple observation - something had to be keeping the Reapers from just attacking the Citadel/Crucible once the Crucible arrived. TIM ran to the Citadel and warned the Reapers that the Crucible was complete and ready to be launched. They KNOW that a massive threat is heading their way. Even if they don't know what it does, the Reapers are far too calculating to let anything that could possibly be considered a threat exist. Soooooo why didn't they just destroy it? The Catalyst was holding them back, that's why. This is made clear when you pick the new "Fuck your choices!" choice and Timmy changes his voice to Harbinger's and says "So be it." At that point, since you made it clear you don't intend to use the Crucible, there was no reason to hold back anymore...and so the onslaught was allowed to commence.
Then there's what should have been a very obvious question for everyone at the end of ME 2. We now know how Reapers are made: an entire civilization is harvested, liquified, and used to make a new Reaper. The obvious question then is "Alright.....so where did the first Reaper come from? What got this whole deal started?" Well thanks to the EC, we now know definitively how it all started. Millenia ago there was a civilization that wanted to solve the "inevitable" conflict between Synthetics and Organics. This civilization created The Catalyst in order to find a solution to this problem. The Catalyst tried various solutions but none of them ever worked. And so it came across a new solution: simply remove the variables. If there are no organics or synthetics to fight each other, then there can't be any conflict between organics and synthetics. Staying true to the theme of "The Created will always rise up against their Creators", so too did the Catalyst ultimately betray those that created him. It was the classic "AI becomes "evil" by following it's programming/purpose too directly."
The way a cleaning robot might kill its master because its master is the one that keeps making the house a mess, so too did the Catalyst remove its creators since they were the ones who were always in conflict with synthetics. So the Catalyst turns the civilization that created it into the first Reaper, "They did not approve", and used that Reaper to begin the first harvest. However, the Catalyst's goal is not the destruction of all forms of life, it's simply to prevent conflict between organics and synethics. As such, organics that are primitive are skipped over so that they can have their chance to advance, live out their lives, and then get harvested. In the Catalyst's twisted logic, this is the perfect solution because he doesn't consider himself as being a great destroyer of life, but rather a great protector of it. Each civilization isn't destroyed, it is preserved in Reaper form. While we consider the Reapers abominations, the Catalyst sees absolutely nothing wrong with them since essentially each one encapsulates the civilization that was harvested to make it.
The point is, SOMETHING had to start the cycle. SOMETHING had to "build" the first Reaper. It was either an entire civilization getting together, drinking their "special kool-aid" from so may humble Dixie cups, then jumping in the liquification vats. Or the process had to be automated and controled by something...something like an AI who feels the best way to fulfill its primary purpose is to destroy all advanced forms of life.
So, in conclusion, say what you want about Space Timmy being a dorky little boy who was formerly known as Spontaneous Combustion Lad. The simple fact remains, though, that SOMETHING had to fill the role that he plays. SOMETHING other than a Reaper had to be there waiting for Shepard. SOMETHING had to have started the Cycle.
Class dismissed.
Alright, we all know that one of the biggest rage-inducing factors of the ME 3 ending was lil' Space Timmy. People thought he was shoe-horned in, that he utterly broke the flow of the game, made certain elements of the game meaningless, etc etc etc. I forget what topic it was in, but I had a discussion with someone regarding this matter and the more I discussed it, the more I came to realize that while you might not like Space Timmy himself, SOMEONE had to fill the role that he filled.
As for complaints about him, I might as well start with this list as provided in a different topic by I.Muir
1: Here in lies a clash of opinion as there's nothing to prove what I'm about to say, however there certainly is nothing that disproves it either. However as the saying goes, "The simpliest explination is often the best one", I will keep things simple. Perhaps he just wasn't around back then.I.Muir said:
Timmy specifically describes himself as the collective will of the Reapers. It could be argued that unless the Reapers are awake and active in the galaxy, Timmy doesn't exist. That is to say maybe when it's said "Sovereign must go activate the Citadel relay", what he's really doing is waking up Timmy who in turn activates the relay. While ordinarily this can be done remotely, the Protheans altered the signals that are being sent and received by the Citadel, thus making it impossible for Timmy to be awoken. They accomplished this by returning to the Citadel only once the Reapers had left the galaxy and thus Timmy would be "asleep" again.
If that doesn't work for you, who's to say that Timmy has any control over the Citadel at all? Where did that theory come from? All we know is that he is an AI that is housed inside the Citadel, it's never said that he has full control over it's functionality.
2: (And this ties in with the "shoe-horned in" complaint) The Reapers have always had motivation, they can't stop talking about it. "We represent order, you represent chaos. We bring order to the chaos of organic life. Your mind cannot possibly comprehend the magnitude of our purpose. Yadda Yadda Yadda". Furthermore, if you were paying attention to the conversation with the Prothean VI on Thessia, you'd recall the VI stating that the Reapers are a symptom (or something) of the cycle, however they are not the controlers of it. They too are just pawns. "So who's really in charge"? We don't know, "It's presence is inferred rather than observed." Right there he tells you that there's something bigger than the Reapers that's controlling them. It's what we in the writers' community like to call foreshadowing. That right there, beyond anything else, refutes the "shoe-horned in" complaint as they tell you about him earlier in the game.
As for the motivation, like I said, you couldn't keep them from talking about it. "We are your salvation through destruction" - Harbinger, ME 2 gun-fight taunt. Well it seems the Reapers were certainly right about one thing: we cannot possibly comprehend their motivations. That...that actually directly answers the "Yo Dawg!" motivational poster pic saying "I heard you didn't want to get killed by synthetics so I'm going to kill you with synthetics!" Again, for those in the class that were paying attention, this is something that was addressed in ME 2, and even in your conversation with Soveriegn back in the first game. Surely Harbinger HAD to mean something when he said lines like "We are your salvation through destruction". But this is a fact that apparently very many people either didn't or couldn't understand. The reason "I'm gonna use synths to kill you so you don't get killed by synths" makes no sense is because it's not supposed to...hence "You cannot comprehend our motivations". The Reapers had been preaching it all along.
3: As for how each ending ACTUALLY plays out, here are my thoughts on the matter:
Green: Shepard's DNA is converted into energy and mixes with the Catalyst's (Timmy, who is an AI) ennergy and this combination is sent out in a shockwave that encompasses the entire galaxy. Synthetics become more organic and organics become more synthetis, thus creating a nice (boring) utopia.
Blue: Everything that Shepard is, his/her will, memories, thoughts, everything, is absorbed into the Catalyst, thus making Shepard the new Catalyst. If you notice in the EC, the Citadel doesn't actually fire a beam for this ending, rather it just sends out a shockwave. This shockwave is essentially a "softwave update" that informs all Reapers that Shepard is now in control.
Red: Destroying the power center sends out what is essentially a hyper EMP that fries all Reapers in the galaxy, as well as - like Timmy says - "damaging technology that you depend on, but it should not take long to rebuild".
In short: Space Magic. I don't see how this is a problem my friends, I've heard plenty of people throw around the phrase "Space Magic" with regards to how the endings are bullshit...well in that case the entire series is bullshit as it revolves around space magic that allows people to travel faster than light, move things with their mind, and fire guns that shoot at relativistic speeds without their arms getting ripped off.
4: With regards to the Illusion of Choice. This doesn't necessarily have to do with Space Timmy, but rather the fact that many people would have prefered having your ending being dictated by the choices you made rather than you being able to choose the ending. When you stop and think about all the major choices that you make throughout the game...what is their end purpose? What is their overall goal? When you stop and think about it, with the exception of some "spur of the moment decisions" like killing Wrex on Virmire or destroying/keeping the Collector base, each choice is ultimately building towards the united galactic fleet and/or building the crucible. That's where you get to see your choices played out. Make good choices and your fleet is huge, make back choices and your fleet is small. Killing the rachni means they're not there to help build the crucible. Saving the geth mean they're available to join your fleet and help build the crucible. Betraying the Krogan mean the Salarians join up, saving the Krogan mean they don't. All the choices in one way or another affect the size of your united galactic fleet. While I do agree it would have been nice if said fleet had a bigger impact than just a few cutscenes (I had heard something about them originally wanting there to be a tactical section where you had to issue orders to the fleet and ground troops, kinda like picking teams at the end of ME 2), in the end your choices are building towards either helping or hindering the fleet.
5: This one actually segues perfectly into where I was going to go with this after having addressed the rest of the list: "Why Space Timmy?"
Which brings me to the thesis of my defense, if it wasn't Space Timmy, SOMEONE had to be there to guide Shepard. Beyond the fact that, as I had already mentioned, the Prothean VI foreshadows the fact that there's something controling the Reapers and the Cycle, there had to be something other than Harbinger sitting there at the end of the story. "Why the hell is that? I wouldn't have minded having a conversation with Harbinger and making a choice, why the fuck is it some ghost kid?" Simple. Would you REALLY have been satisfied with a conversation with Harbinger explaining the various uses of the Crucible? Harbinger, Mr. "OMG LULZ ORGANICS ARE WORTHLESS, INSIGNIFIGANT ANTS! THEY THINK THEY CAN WIN WHEN I HAZ CHEEZBURGER?!" Well, you get the point. He goes from gloom and doom, you're wasting your time, your struggle is futile, to all of a sudden saying "And if you choose THIS option you can REALLY fuck us in the ass by enslaving us and making us obey your very will. Oh all that talk about you being worthless? Well I honestly didn't think you'd get this far..." It would be a HUGE character shift for Harbinger to go from explaining how everything you do is meaningless to explaining the various ways you can utterly defeat the Reapers via destruction, control, or pacification.
"Hey douchefag, you STILL have said why the role is necessary." Hold your nuts, I'm getting to that. For starters - and a rather simple observation - something had to be keeping the Reapers from just attacking the Citadel/Crucible once the Crucible arrived. TIM ran to the Citadel and warned the Reapers that the Crucible was complete and ready to be launched. They KNOW that a massive threat is heading their way. Even if they don't know what it does, the Reapers are far too calculating to let anything that could possibly be considered a threat exist. Soooooo why didn't they just destroy it? The Catalyst was holding them back, that's why. This is made clear when you pick the new "Fuck your choices!" choice and Timmy changes his voice to Harbinger's and says "So be it." At that point, since you made it clear you don't intend to use the Crucible, there was no reason to hold back anymore...and so the onslaught was allowed to commence.
Then there's what should have been a very obvious question for everyone at the end of ME 2. We now know how Reapers are made: an entire civilization is harvested, liquified, and used to make a new Reaper. The obvious question then is "Alright.....so where did the first Reaper come from? What got this whole deal started?" Well thanks to the EC, we now know definitively how it all started. Millenia ago there was a civilization that wanted to solve the "inevitable" conflict between Synthetics and Organics. This civilization created The Catalyst in order to find a solution to this problem. The Catalyst tried various solutions but none of them ever worked. And so it came across a new solution: simply remove the variables. If there are no organics or synthetics to fight each other, then there can't be any conflict between organics and synthetics. Staying true to the theme of "The Created will always rise up against their Creators", so too did the Catalyst ultimately betray those that created him. It was the classic "AI becomes "evil" by following it's programming/purpose too directly."
The way a cleaning robot might kill its master because its master is the one that keeps making the house a mess, so too did the Catalyst remove its creators since they were the ones who were always in conflict with synthetics. So the Catalyst turns the civilization that created it into the first Reaper, "They did not approve", and used that Reaper to begin the first harvest. However, the Catalyst's goal is not the destruction of all forms of life, it's simply to prevent conflict between organics and synethics. As such, organics that are primitive are skipped over so that they can have their chance to advance, live out their lives, and then get harvested. In the Catalyst's twisted logic, this is the perfect solution because he doesn't consider himself as being a great destroyer of life, but rather a great protector of it. Each civilization isn't destroyed, it is preserved in Reaper form. While we consider the Reapers abominations, the Catalyst sees absolutely nothing wrong with them since essentially each one encapsulates the civilization that was harvested to make it.
The point is, SOMETHING had to start the cycle. SOMETHING had to "build" the first Reaper. It was either an entire civilization getting together, drinking their "special kool-aid" from so may humble Dixie cups, then jumping in the liquification vats. Or the process had to be automated and controled by something...something like an AI who feels the best way to fulfill its primary purpose is to destroy all advanced forms of life.
So, in conclusion, say what you want about Space Timmy being a dorky little boy who was formerly known as Spontaneous Combustion Lad. The simple fact remains, though, that SOMETHING had to fill the role that he plays. SOMETHING other than a Reaper had to be there waiting for Shepard. SOMETHING had to have started the Cycle.
Class dismissed.
