Are people using a mouse and keyboard going to play against people using a controller? The results will be hilarious (in that anyone using a controller wont stand a chance). Everyone will be forced to use a mouse.
There have been games that have released with this allowance. They achieve balance by offering powerful aim-assist to the gamepad players.
In any other context that would be the foulest cheating... but in the context of game-design for vastly different interfaces, it's a perfectly reasonable handicap.
You wouldn't be able to (and can't) do this on competitive PC/Steambox games though because you can just fake it on PC, so gamepad players would never be able to compete.
In any other context that would be the foulest cheating... but in the context of game-design for vastly different interfaces, it's a perfectly reasonable handicap.