1. If we're ever going to play protected video (DVDs, HD/BD) then we have to protect the decoded bitmaps all the way to the frame buffer, and we can't do that in an open driver.
2. For workstation business we invest a lot of money in performance-related driver work and wouldn't want to open source that because some of that work *is* useful to competitors.
Since we're not playing protected video today we could convert to use more open source in the short term, but by the time we finished that it would probably be time to start converting back
IMO the best strategy is to have open AND closed drivers and use each one where it fits best.