Why do we create skybox textures the way we see them everywhere ?

Ok, Skyboxes cover all sides. It is nice to have an option for spaceships and also flightsimulators etc. For these applications skyboxes have to be made the traditional way of course.

However I ask me, is it really a good way to make a skybox for a racing game or a game where characters stay on the surface of a terrain ? What I mean is, a skybox mimickes a full sphere, while actually a dome would be sufficient for most uses with terrain.

Points is, if we make a skybox that has its sides with only texture on the upper part of a square and the lower part in alha or black. these textures use roughly half the memory with compression. And the bottom is not needed at all of a skybox. So basically a way to mimick a dome rather than a sphere.

So my actual question is, is this a valid option to help with performance for non flight / space games ?