On this day in 1521, at the conclusion of the Diet of Worms, the Edict of Worms was issued declaring Martin Luther an outlaw and a heretic. The effect of the Diet of Worms on the participants' intestinal tracts is not mentioned. (Sorry)
I once worked with a heretic; he had the documents to prove it. While attending an Episcopal (as I recall) seminary he had disagreed with the church over some point of doctrine that frankly made my eyes glaze over when he explained it to me. I remember thinking (1) Churches still actually declare people to be heretics? (Of course it doesn't have the ramifications it did for Luther.) (2) How cool that he has an official certificate. If it were me I'd definitely hang it on my wall. (3) There's a good conversation-starter at parties or an item for the resume.
What really struck me was the obscurity of the disagreement. I can't remember it's exact nature, but it was well beyond the level of a typical churchgoer. I think this is a partial answer to the question of why people stick with certain churches; most of the doctrinal differences are minute enough that people don't see the point of giving up the social network they've built up.
I imagine many people would consider a lot of the disputes that caused the formation of the various Protestant churches to be silly. I think most parishioners just don't worry about such details, or maybe they don't want to know. They might be unpleasantly surprised to find themselves thinking like heretics if they looked into their church's doctrine.