Of course they could do both, but isn't Nouveau 3D performance so far behind the proprietary drivers that nobody is going to be using it for gaming anyway (at least for a long time)?
I guess it could be that they're just buying some goodwill from the general community in anticipation of working closer with it.
One of the main reasons for the lack of performance is clock/power management of the cards. The proprietary driver clocks up the GPU while nouveau does not. I would guess this is one of the reasons for why they asked for the power management documentation.
Not really that bad. On Kepler+ cards it sucks, but I threw Nouveau on my old gtx 285 pc back before I realized Nvidia were anti-opensource assholes (tbh, just like Intel is with cpus) and it can run most games at 60 fps at 1080p as long as they are running at medium or so settings.
That has to be in context, though, since I was only running minecraft, darkplaces, doomsday, flash games, and humble bundle games like Torchlight and Bastion on it for my mother to play. But you can enable reclocking, and it does run ok, but it is around half as fast as the proprietary driver is in the same games at best.
This is pure conjecture on both our parts, of course. I was simply taking issue with your prediction of Nouveau not being useful for a 'long time'. It almost sounded like you were suggesting this was a useless gesture on Nvidia's part.
Seems like they just want Nouveau to work better during normal desktop usage. I doubt they plan to release enough information to make them competitive with their proprietary drivers.