Or if you join the war without call to arms event, you couldn't become coalition leader. In case of a request there is always a risk that these morons would reject it for unknown reason.
I mean in theory there is nothing what could prevent you from join the war. As long as you are ally, you have an automatic and absolutely legal reason to join the war. Even if your allies don't want it, it wouldn't hinder you. You just send your troop to the front, maybe then you don't coordinate your actions with your allies (what you don't do in EUIV anyway) or couldn't make the decision regarding peace negotiation for the coalition (don't be a coalition leader), but there is absolutely no reason why you can't declare war on the enemies of your ally.
Wasn't there a Casus Belli in EU3 to make a declare war against a nation at war with one of your allies? If allowing unlimited joining of your allies' wars raises too many complications, could we bring back something like this?