Are you sure it's a weight thing? Since paint isn't that heavy anyway, I would suspect the vinyl that the wrap is made from would be comparable if not heavier. Using a wrap probably just makes getting all of the sponsor decals on and neat easier, plus fancier graphics can be done really cheaply because the vinyl is just printed out. For a real painter to do all the fancy paint work would cost a lot more, and not be as consistent.

What about the weight of the solvent? That's going to evaporate.
I agree that in most cases it's ultimately aesthetics.