The "nebula thingy" was just a test image (to help figure out the problems of getting OGL and D3D to be able to use the same cubemap files and show them the same way), so it was never really intended to be anything but crap.
The problem is that no one ever bothered to make a new one and instead just included the original test version in the MediaVPs.
Oh, and the default envmap (cubemap) will continue to get used if your video card doesn't have the required support to create one automatically for each mission. It's only if your video card has the required extensions to create the envmaps that the mission envmap will override the default one. Technically, every mission should include a pre-generated envmap and have that be used instead, but I appear to be the only person doing that.