A few months ago, Microsoft released a paper about GPU Hang Detection and Recovery in which they state that "Microsoft OCA data shows that 20 percent of all Windows crashes are due to GPU hardware hangs or instabilities and that the GPU is the largest device category reporting crashes (ahead of categories such as network adapters, hard drives, and USB cameras)".
It may well explain why Apple wants to keep a close control on what graphic cards are available and doesn't allow third parties as it would dramatically increase problems in OS X.
Funny that you mentioned that as the recent iMac GPU trouble only proved that Apple doesn't have a clue when it comes to GPU. Freezing issuses, poor choice (wth, even MBP has better GPU) and MP......... insane. They should either fire somebody there or hire someone more qualified.