The hardware universe is unfortunately a little broader than just nVidia, intel, & ATI/AMD. Via/S3 still produce embedded graphics chipsets as does Imagination Tech(PowerVR).
The PowerVR based GMA500 chips that Intel is selling right now have rather poor drivers, and the prospects for improving them are quite poor as documentation won't be released and nouveau style reverse engineering will take quite a long time.
Via has released some documentation on their chips, but from cursory observation, the OpenChrome folks don't have much in the way of developers to work on getting the drivers up to snuff.
Nouveau itself is improving, but by the time they can offer solid support for the chips that are shipping now, nvidia will have released their new DirectX11 chips that will likely require a substantial amount of new reverse engineering work.
Virtualized environments are just now starting to get decent 3d support, and properly working compositing in virtualized environments is still a ways away.
This is not to mention there are situations where disabling compositing is desirable because of problems that applications experience when they are composited. For instance, many windows applications running under wine, crawl to a halt under compositing and lord knows if those problems can ever be fixed.
My point is not that Gnome3 shouldn't be designed to heavily take advantage of compositing, but that it if it does, it should provide some graceful fallbacks that doesn't totally gut the functionality of the environment when compositing doesn't work as expected.