Yes . You are right, but also as far as I remember if You own a castle, You are unregistered for all other castles (I might be totally wrong, sorry if it is the case) and You can not help Your ally to fight on another castle without risking of losing tons of exp. So You have come up with tricky ideas to not lose it. For example, register some dwarf with golem on alt clan, move your castle there, then fight together with Your ally and then retake castle back on your main clan or something.
NEVERMIND I JUST READ THE CHANGELOG