A casual person would say yes (for obvious reasons), but this me your reading. So I say it depends, think about it, having many personalities can be very beneficial with many things, acting is one (obviously), work, and a spy (maybe).
There's more benefit's to it, but can't think of much more at the time, I'm a bit of a airhead heh..
So I leave it to the reader to think of many more possibilities of having more than one personality, mind you it's only beneficial if you can control these personalities like an actor, if you can't that's called personality disorder of course.