Most first person shooter PC games, sure, but there are thousands of PC games that assume you have a mouse and a keyboard. Point and click adventures, that sort of thing.
Even some games you'd think should work well with a controller don't really. Unepic's a 2d side scrolling action-adventure game, you'd think it'd be controller friendly, and there are console ports of it, but the PC version uses 10 hotkeys for various spells and abilities. I managed to map i think 4 or 5 of them to controller buttons, but it meant more going through the game menu and reassigning spells than it would have using a keyboard.