It seems to be mostly older Intel drivers causing issues. Does everything work with the current git master?

The main reason they want to start using OpenGL2 is so they can port KWin to OpenGL ES and run on mobile devices. Sounds like FBOs are one of the main OpenGL3 features they'd like to use, but they will retain backwards compatible code for it. Other stuff like geometry shaders might be used in advanced effects only, or with alternative lower-quality backups in place.