Yeah, that works for some subset of games. If you're doing is playing FPS games and some workable subset of side-scrollers, sure. But games designed for a specific control scheme generally play much better with that controller.
I've played my fair share of emulated NES/SNES games, and while workable on a keyboard, let's not kid ourselves and say it's just as good.
And I remember playing DOOM not with WASD (because I don't think anyone was at that point), but with the numeric control pad so I could easily strafe (when the default most people seemed to use were arrow keys), so it's not like WASD came out fully formed and obvious from the start.
Control schemes change, and people usually end up adopting what works best for a specific genre (or specific game, if it's genre-bending enough).
Don't get me wrong, I abhor the idea of playing a FPS game with a controller as well... but not all games are FPS games.