Excommunication is great if used correctly, because it tends to commite the AI of the Cath factions to serious border fights. Which means that if you play a "calculated attrition" game, and are sensitive to which provinces have real strategic value *cough*Silesia*cough*, you can often tempt the opponent to invade, only to have to monkeystomp them with six stacks they didn't think would be available. If you've read your map correctly, you should be able to consolidate and completely pillage some other territory in order to deny reserves to his invasion force (on normal, this is a very reliable way of snapping up his heirs) -- thus setting up the next domino in the chain.