It's semi-required by browsers for the new WebGL API. You can emulate OpenGL ES over OpenGL, although the shading languages are slightly different so it's just a lot easier to have a proper OpenGL ES implementation. The shading language for ES was a little different than regular GLSL, in particular, and I don't think they aligned until GLSL 1.40 or so which Mesa doesn't yet support.
Originally Posted by bridgman
Yeah, good point. Some browsers require GL ES for WebGL, others implement it over regular GL, but the trend seems to be towards pushing the work down into the driver stack.