I just use a Keyboard and mouse for playing PC games on my computer. But, if I was in to flight simulators, racing games, etc, I probably use a controller, or joystick.
I only use keyboard and mouse because I don't really want to buy more gaming accessories that would collect dust in the corner of my room. If PC gaming released more flight sims or driving games then I'll be prompt to purchase a joystick.
I think using a controller for fighting games on the PC might be better as well since there are so many keys on a keyboard which makes it confusing sometimes.
I agree using a controller for a PC is very good for playing Emulated games, ROMs, and PC ports of Console games since the game was made more for playing with a controller.