well it depends on your method of play really. with free investure you can name your own antipope and protect yourself from the banhammer of rome. However in that case you can't request excommunications on characters outside your realm or for invasions. You do however keep your ability to join in crusades, gain a claim, for your antipope, on Rome, and also have a way to disinherit your male children with the exception of your heir by nominating them for bishop. one major drawback of an antipope is Moral authority starts to degrade for as long as you have an antipope under your thumb.