video drivers, agp, mtrr, and write-combining

I have a silly question. I've noticed that some, though no all, video drivers add their video memory to MTRR as 'write-combining' if both MTRR and AGP are configured in. Is there a reason that all video drivers don't do this? Is it all would benefit from write-combining memory, but that some

video drivers, agp, mtrr, and write-combining

I have a silly question. I've noticed that some, though no all, video drivers add their video memory to MTRR as 'write-combining' if both MTRR and AGP are configured in. Is there a reason that all video drivers don't do this? Is it all would benefit from write-combining memory, but that some