By releasing updates on a regular basis, they keep customer annoyance to a low. Take for example: You love UT, but you can't play it due to some D3D compatability problem with your ATI card. ATI has fixed the problem, they however choose NOT to release the drivers. Instead they wait and wait and wait, untill they have solved ALL known issues. Which, would probably take them forever and a half to do.

And nVidia can write drivers? ... definitely not for last 12 months ...