NVidia have their own high-quality OpenGL implementation. Isn't that better?
Driver support going away after a few years, random issues, etc
Every single nVidia card I used under linux just worked TM (ofc I'm not "pure" because I really couldn't care less about open vs closed drivers)
It is strictly worse. NVidia not only has no interest in having a Mesa driver, but would prefer if they didn't have to compete with Mesa at all.