Sean Hollyman said:
For example, when playing Halo 3 Legendary co-op, I just stood behind and acted as the spawn guy, while my friend ran ahead and fought. I got the legendary achievment afterwards, even though I did hardly any work.
Fine, it's your game, but with this line of thinking you'll never
actually get the same level of satisfaction that you get by beating a game legitimately.
Also, how do you plan on beating the game on Legendary solo? Sure, there's not an achievement for it, but the game still keeps track of what levels you have beaten on what difficulty and whether you had help or not. Oh, yeah, and I'll be impressed when you beat Reach on Mythic difficulty using nothing but a plasma pistol and bubble gum.
EDIT: By the way, in most games it is the journey that matters, not the ending. If you play games just to see the ending or just to hear that "baboop!" sound of an achievement, then