I appreciate the other answers attempting to claim the keyboard/mouse fanboys are being unreasonable and obnoxious… but clearly you aren't a gamer if that's what you think. read more
There are games where controllers are king, most games can get away with either, but FPS games must be played with a keyboard and mouse when allowed. I forget which game they tried this on, but some developer tried to make a game such that PS3 and PC players could play together -and I think it was an FPS. read more
For some games, using a controller just plain works better than mouse and keyboard, which is why many pc gamers (like myself) own a usb controller and use it occasionally. However, not everybody does, and there is not much worse than a game being basically unplayable because you don't have it. read more