I don't think it'll ruin the game, I just think it's unnecessary. I've played some Xbox 360 ME2 and I have to say that PC controls trump on every angle.
Everyone I know who's played both versions also think that PC controls are far greater than the controller based ones.
Reasons why:
- Because more options is never a bad thing. They have to make the 360 controls for 360, and bringing them to PC is an extremely simple porting process because Vista/Win7 support it natively.
- Because some people game with their PC's on a couch, and using keyboard/mouse on a couch is difficult and uncomfortable.
- Some people enjoy using gamepads more, regardless of the added precision of the mouse, for whatever reason (some people were just raised on gamepads and never got used to K&M, some people find K&M uncomfortable but gamepads to be easier on their hands or whatever)
Basically, there's nearly zero effort required to do it, and it will benefit many gamers. Personally I would rather swallow a bullet than play a shooter on PC with a gamepad, but I also recognize that I am not everybody, and there is no point disadvantaging some people for no good reason.