I actually find them beautiful. Personal preference, I guess.
The new ones with Entrenchment are truly a sight to be hold.
What it comes down to is, just pure black space with stars is boring. So skyboxes let you imagine that every map you play in is smack dab in a nebula of some sort. Course, since planets dont orbit and there can be 50+ planets per star, realism doesn't really belong in a game like this. It's all about style, and that's something this game has in spades. The ship designs, the abilities, all of it is highly improbable and unrealistic, but still very captivating and imaginative and helps the game stand for itself among similar RTS games.
It's similar to the Homeworld series in this regard. It almost hints that space is just another planetary environment, perhaps that our home is actually among the stars themselves. I find the color-altered pictures of NASA and the like of nebulas to be highly interesting... and inspiring.