To me, skyboxed starfield looks a lot better than the retail stars for the following reasons:
1. It has more stars on it. The maximum amount of stars allowed in missions in FRED is 2000; I don't know if you can boost the value in notepad, but the amount of visible stars is somewhere around 5000-6000 depending on what you choose as the threshold magnitude (which varies depending on eyesight and conditions).
2. It has more variation in star brightnesses (and colours as well).
3. Default stars can not be used in conjunction with planet skyboxes at all (they are rendered on top of the skybox, so you see the retail stars through the planets...).
4. It doesn't have crappy looking motion blur.
Combining skybox starfield with nebulas looks much better than just using retail stars with exact same nebulas. Not only that, but I find that skybox starfield gives you better sense of motion when turning the ship.
I'll agree that background is not the most important or critical piece that makes or breaks a mission for me, but it certainly helps to improve the look and feel of the mission because the background is everywhere while ships and other background elements only occupy parts of the background.
Even if you don't look at the background when fighting, screenshots will look much better when the background is nice...
