Yes and yes.
It is quite prevalent amongst the less computer-literate users, it is shocking how many have no idea of the nastiness that is loose on the internet. Yes, AV should find 99%+ of the key-loggers but many, many people don't have any AV software. And if they do, they don't bother with a subscription. "AV? Yes, I got that with the PC" (2 years ago)
Keyboard loggers can be very small and efficient.
Yes, Linux would stop of lot of it! But do you think you could persuade people to actually do this?