Ok, I'm not going to wade through 7 pages of replies, I'm just going to respond to the first post. I apologize that it's going to be so obscenely verbose, but I take self-defense rather seriously. I'll start by addressing the three points in that post.
1. This depends on where you are. Different places have different laws governing when you can use a firearm. I will assume you know what you're talking about in regards to the laws where you are, and base the rest of what I say on that assumption.
Now, another thing you need to consider is that even the possibility of having a gun drawn will make most people want to get away. If you even act like you may do so, most people will want to avoid you. You also need to consider that if someone is in fact a violent criminal, they're unlikely to go to the police if you do pull a weapon on them. They hate cops. They fear them.
You mention possible criminal charges for doing this, though, and that is worth discussing. I'll just ask you this: would you rather be judged by twelve or carried by six? I'd rather be prosecuted for drawing a gun in a situation where it is not legal than killed because I didn't.
2. If you're like me, you make a big deal out of situational awareness. You look around every corner. You take any opportunity to glance over your shoulder. You keep an eye on the shadows of people behind you. You watch people for any sign they might be up to something that could potentially threaten you. When strange people approach you, you think about the best way to take them down if they try anything. Basically, you NEVER let your guard down and you NEVER assume you are safe until you're home, the door is locked, and you have established that nobody else is there with you unless you want them to be. Paranoid? Sure. But it also means I'm very, very hard to get the jump on. Even if I don't see someone trying, this appearance of alertness is discouraging for muggers and the like when they could go for someone who looks like an easier target.
But let's say someone does manage to sneak up on me with a gun. If that person intends to kill me (which is possible when the local gangs are doing initiations) I'm fucked. It doesn't matter whether I have a gun or not. But if they want something else, they'll wish to avoid actually killing me because doing so means more police attention, and a longer sentence (or execution) if that police attention lands them in court. In this case, I have a chance to do something, if I feel it is necessary to protect my life. I am quite capable of fighting back if someone has a gun to my head, though I hope I'll never have to. Which leads us to that third point...
3. Your stuff is replaceable, but being a gamer is the closest you'll get to having a replaceable life. You don't get to hit the quickload key if someone blows your head off in real life (I HAD to make this game-related somehow). You can get a new phone, you can get more money, you can get a new credit card, you can get a new ID. You cannot get a new life. If someone wants to mug me, violence is a last resort. If the guy takes everything I have, I don't WANT to chase him. I want to get in contact with the police ASAP, and let THEM handle it. If they don't get the guy, that sucks, but I'm still breathing.
Now, I've got some points of my own:
1. Having a gun gives you an advantage in some very dangerous situations. It does not make you safe. It does not necessarily even make you less unsafe. But it might, and that counts for something.
2. Despite what all those cop shows and law dramas would have you believe, police don't try to go after every criminal act to the fullest extent possible. They don't have the time, money, or manpower to do so. Of course if someone draws a gun on some potential bad guys and they report it, the cops have to do something, but they may very well decide it's not worth a great deal of their time and effort when they have a bunch of other crimes to investigate. If some shady looking guy says you pulled a gun on him in a manner that wasn't legally permissible (assuming he even knows whether it was or not) and they go to you, and you say that you felt threatened (or even lie and say nothing happened at all), it wouldn't be surprising if they just wrote it off after a brief investigation.
3. If your roommate carries a gun because he's worried about muggings, he's being foolish. You don't carry a lethal weapon to protect yourself from muggings, you carry it to protect yourself from battery, rape, or murder. You carry a gun to shoot people who want to kill you, not to shoot people who want you to hand over your stuff.
Hope that's insightful.
EDIT: Forgot a really important fourth point: There's no reason you can't tell someone you're carrying a gun. If some guy wants to do terrible things to you and you say "Stop! I have a weapon!" While reaching for your holster, he's going to think twice about doing anything that might upset you further. Plus, giving a warning makes you look more like the good guy in a police report.