On newer nVidia hardware you can indeed do a lot of stuff in the firmware, but that firmware is not free to redistribute like AMD's firmware so the open-source developers have to write it again. That way it doesn't really matter (from a user's point of view) if code is in the driver or firmware.
Power management is the reason I'm stuck with Windows XP and Debian LENNY with drivers dating from 2008-12 to 2009-03 (although I'm quite happy with both of them). Yes, I have an "old" notebook with X1600 mobility, but it does everything I need so why upgrade?
The first GCN card (HD 7970) has only been in stores for a few days, and it's the high end card which I wouldn't expect to see paired with the current open source drivers anyways.
Don't forget the people who use Windows as game loader, and boot into Linux to do real work. They have little reason to install the proprietary driver as long as a somewhat working open source driver exists.