There is a simple difference between having multiple personalities and having multiple personality disorder. All superheroes like Batman (The Dark Knight is awesome: super entertainment package that appeals most), Spiderman, Superman etc. have multiple personalities that they maintain in an orderly fashion. It's when they cannot handle them in order that they run into trouble.
Many of us also have several moods. Having moods is not a bad thing (in fact if we don't have moods life becomes trivial) but having them out of order is not desirable with limited resources around. Mood distribution and personality of a person are correlated. Having strong moods gives you some sort of extra strength but "strong moods come with strong responsibilities".