I think the idea of ducal independence is that a duchy could declare war on a duchy in another realm, and go fight them--an actual battle between units, and you don't have to wait for the king to decide he wants to war.
Exactly. A federation would kill that. The entire point of ducal independence is having dukes fight against each other over insults, small pieces of land, or just plain boredom (anybody recalls the Darka-Eston friendly war? Good solution, in my opinion).
