And how does that make sense?
Like... if you have a fork, does that mean that you'll never buy a spoon?
Your comparison makes no sense. A better saying would be if I had a spork(the mouse and keyboard) would I not buy a fork(the controller)? and hell no I would not buy the fork I got a perfectly good spork that does other things (like lets me eat soup) and stab my food with it. The fork would be a waste of money and only let me do one thing while the spork has more options open to it.
If you're an avid gamer and use your PC for gaming, a controller can be a really cool thing. And in todays world, if you have one of the consoles, you can reuse your controller without a problem on your PC anyway.
Yeah they might be cool to play with but they should never be necessary to play a game. You would alienate a bunch of possible players just be requires that they use a controller. And what if i dont have a console to use them with? Im supposed to spend a bunch of money (probably more money than I paid for the game) so that I can play it Fuck that.