Small doses.

There are a great many characters in books or movies or whatnot that people say should have their own.  I don't think these people have necessarily thought this all the way through.  What if the reason the character is loved so much is because they're only present for a short time?  What if, given a full length book, movie, or whatever, it becomes clear that they're not the best character after all?  That they're gimmicky, or not that good of a person, or a jerk.  True, every time they appear in the book, movie, or whatever, they risk taking over, but given the spotlight, the veneer might come off.  Plus, if the character isn't exactly how people have built them up in their minds, they'll hate them, or say that whoever made the book, movie, whatever screwed up.