As gaidax said, right now its not "Bring the player not the class". It's "bring the correct legendary not the player". A week ago i got the destro bracers and Im doing about 100-150k more dps on heavy cleave fights then before. So much for legendaries are only minor dps gains etc. Same goes for fire mage bracers ....
You have a good demo legendary? Go play demo everywhere. You have the affliction Head? Gratz go play affli everywhere.
To your question what specc you should play on Cenarius/Xavius:
I would look at your raid and see what you can bring to it to upgrade it. If you play with 5 mages you dont really need the cleave since ignite will do most of the work -> play demo
Your raid lacks cleave -> play destro
For Xavius goes pretty much the same. Last phase is whoring phase ... doesnt matter which specc you play, but since you have to kill the boss and demo is the best ST specc for warlock ....
(i play destro ^^)