This might be a silly question, but isn't MESA here just for it : providing openGL functionality to sytems that don't have good hardware? And wasn't it planned somewhere to split together wined3d and winex11 (not related to the question here, but just curious :) )
Gentoo users still have to set the +opengl use flag... That's where most of the complaints came from.
The other aspect is that even software GL isn't available when you're using Xvnc or similar servers.