Use the right tool for the right job. I don't try to knock a nail in with a spanner, I pick up the hammer. Therefore:
Flight sim -> Joystick & keyboard
Racing -> Gamepad (or steering wheel if you've invested)
Anything else -> K&M
And it just so happens that I hate racing games so I never bothered with a 'pad.
From a historic point of view, truly old games were often simpler and didn't need many controls (see Earthbound & FFVI era - 4 direction buttons, "use", "back", "menu" - anything else is gravy).
In modern games K&M is considered the superior choice for FPS in particular due to the twitch-aim responsiveness (which isn't to say 'pads can't twitch aim, but it's much easier with a mouse). It is also better for top-down strategy 'cuz mouse (duh), and clearly anything where a lot of different keybinds would be useful benefits from a keyboard too. Saves on ridiculous crap like Mass Effect 2 having "sprint", "take cover", "break cover", "vault cover" and "use" all bound to one button (which wouldn't have been so bad if they'd allowed PC users to separate at least some of these). This, incidentally, is the main source of my resentment towards gamepads - PC ports' interfaces are horribly clunky and dumbed down "streamlined" because devs can't be bothered to fix the interface to fit the much more powerful and versatile K&M setup. Which, admittedly, is more an issue of lazy devs ports than a legitimate problem with 'pads themselves.
The only real advantages of a 'pad is that it is compact (which is why they're used for consoles) and the analog sticks/buttons are good for variable accelerator control and fine steering. These, in my opinion, do not begin to make up for the loss of versatility, twitch responsiveness, number of keybinds, etc. save in a very small selection of games (ie the relatively small racing genre, which I don't enjoy anyway). In short, gamepads are a necessary evil of consoles, but PC users aren't forced to put up with them.