Pretty simple, really: is there any reason I would want my country to embrace a particular heresy? It seems like a super-bad deal. All the orthodox (as in non-heretics) countries get to declare Holy War on you and all your vassals hate you. Since the only event I've seen which makes you a heretic doesn't really hurt you not to convert (it imprisons your chaplain, but since it's an event imprisonment, you can immediately release him for +10 opinion from all vassals), it's doesn't seem like there's any reason at all to become a heretic.
Am I missing something?
Am I missing something?