All my aliens are wearing their disguises in public. When they're at home alone or with their families (or alien friends) they change to their original alien form. Also if there's an alien night at the bar they will be on their alien form.
The Sim on my avatar is also an alien and she's almost always on her disguise.
Aliens wearing disguises suits on my play style, because I like to think that they don't want to reveal themselves to everyone. In my game it's much more interesting this way. So yes, there is a very big point of alien disguises.