Well, I didn't let him out, but haven't replayed the part to let him out to see the real difference but I can tell you what happened when I didn't let him out...

Behlegar went "nnoooooo come baaack! come baaaaackkk!" and the other rhyming mage whos name I always forge gave my dragon form a fireball attack on a decently long cooldown timer that exploded in a large radius on impact, usually wiping out entire islands of towers in the end run of the mission, and the divine was not immediately freed but ended up in the ending cinematic anyways... and that's all I remember :P