"I have to wonder what the Old Gods got out of dominating the Elemental Lords, though"

Well I always presumed that since the old gods lived to so long ago, on a primordial azeroth, that there wasn't much else to dominate and control. This is before (or after, lore kinda gets fuzzy) the Titans came and created the earthen, giants, robo-gnomes, vrykul etc and before they created the well of eternity (so no elves yet either), so the old gods didn't have much to work with, it was either dominate the Elemental Lords or don't dominate at all. Remember that when the Silithus appeared C'tun was quick to enslave/manipulate them, so I imagine its more a issue of availability (since evil=domination they had to dominate something/one).

"Would the Old Gods even need someone to break them free "

Always wondered about that one myself, aren't the old gods technically immobile, you know got no legs and are literally rooted into the earth. Even if they did break out, its not like they can go anywhere.
So its probably a figurative 'free' as in 'allow my evil and influence to wash over the world unhindered'-type thing.