but games like GTA and Mafia that mix driving with action and shooting I find better overall on controller, as do many others.
I'm sure some people do, but not everyone. I've played GTA with a controller, and I've played it with KB+M. I would take KB+M every time. Part of that, no doubt, is the fact I rarely use a controller, and more often use KB+M. But that's not really the issue. For me, using the KB+M is a superior experience.
PC gaming is about flexibility and confining yourself to just KB+M is neglecting the best aspect of PC gaming - flexibility.
Flexibility doesn't overcome practicality. I have the ability to plug my PC into my TV, but I rarely do so, because the computer and TV are in separate rooms. If a game developer made a game and said "we're going to design this for people using 50" TVs with their computers", I'd rightly be annoyed. Yes, it's possible to have that setup, but it's not the standard, and it's silly to force everyone into that situation.
Some people have controllers for their computers. Many more don't. So designing a game around that as a requirement is foolish. Make sure your KB+M support is solid, and if you want to implement optional controller support, go nuts. But don't ignore KB+M just because the controller is supposedly "superior".