"what the Old Gods got out of dominating the Elemental Lords"
Endless supply of crafting mats.

[2. Trade] Yogg-Saron: WTS stacks of Ancient Fire - 210g per stack (Better than AH prices)!! /w me!!!
[2. Trade] C'Thun: lawl Yogg u nab u cant sell them the player base isnt there yet
[2. Trade] Yogg-Saron: -_-...
[2. Trade] C'Thun: :)))))

It seems to me that the Old Gods, who value chaos and destruction above all else, would want these powerful elemental forces to do just that: destroy and cause chaos. I believe it was in a previous KYL that the Old Gods had the elementals at war with each other for their pleasure. Even imprisoned in the earth, they continue to sew as much chaos and destruction as possible.

As for whether they want to be freed or not, we may never know. It does seem to me though, that they might be perfectly happy staying "imprisoned" while still having a significant effect on the inhabitants of Azeroth. Yogg's influence over the titans left in Ulduar, for example. He had all of them under his control, yet he did not leave his "prison." Maybe a more indirect influence and cause of destruction is preferred.

Think of it this way: You want to cause chaos and destruction for fun. The titans come along and stop you from doing so, as they value order above all else. You convince them that destroying you would also destroy the world they're trying to save, so they merely imprison you. "Please, don't throw me in the briar patch!"

Now you're sitting snug beneath the earth able to cause chaos and turmoil without getting too much notice. After all, if something were to happen to show that Azeroth was "beyond saving," then the titans might just send Algalon to reorder everything... such as killing Lokan, the prime designate.

Ok, so you can't kill your titan jailors, but you can corrupt and influence them. now they carry out your will and are under the impression is their own bidding. Do you really want to be let loose on the face of Azeroth? Probably not if it gets the titan's attention... or gets it too soon.

Lets say that you're waiting until you've got enough corruptive influence to build a new army out of the titans own creations. Myzrael was pretty upset that you had summoned her forth too early, even though you'd already freed her. Why not just pop out of the ground and start causing havoc? Because you need to wait until you and your armies are strong enough to take on the titans.

Earthen, vykrul, and their descendants are susceptible to the whisperings of the Old Gods. This includes dwarves, gnomes, troggs, and humans to name a few. Perhaps we're the source of the Old God's planned armies without even knowing it. Maybe they're just waiting for one or two really powerful mortals that they can shape and mold into agents of the titan's destruction... like Med'an? He's already heard the whispers of C'thun time and again. Just like Hodir, Lokan, Freya, and Cho'gall the Old Gods are collecting power beings to serve them and carry out their will without having to leave their "prisons." It's a matter of time before Med'an is going to have to deal with C'thun and his corrupting influence, or fall prey to it.

Lastly, we've not seen much of the Air, Water, and Earthen courts yet, but there was not a big reason for them to make themselves known. Ragnaros got lucky enough to be summoned by Thaurissan, or we'd probably not have heard much about him either. In the coming expansion, we've got Vashj'ir, Skywall, and Deephome on the zone lists. After the Aspect of Earth breaks free from the elemental plane, we'll see a much bigger elemental influence on Azeroth.

My prediction: The Cataclysm is going to sew enough chaos and destruction that the Old Gods can finally make their move. The old elemental powers are now freed upon Azeroth and they've collected a large number of followers in the Twilight Hammer sects. They'll try and gather enough of an army to claim Azeroth for their own once and for all. Now that mortals have found and learned the "all clear" code from Algalon, what's to stop the Old Gods from using it to keep the titans out of their affairs?

_____________________________________
www.stackingstamina.com

"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.