The explanation that was given at my dojo is in fact that the Hakama serves to hide ones legs. And it does seem to be at least partly true, because when my sensei wants us to really see how he's standing, he holds his hakama up.
Also, that is the reason beginners don't wear it. The Hakama would conceil errors in stance and movement, so sensei/sempai can't correct you. That and the cost as was mentioned above.
[EDIT: Stupid spelling mistakes