My gut feeling would be that with an incredibly mobile group such as the warlocks a healer running with them might fare better. I imagine DK's would want to be involved in a fight more, leading to more vulnerability for the healer.

Constant running into range, dotting everything up and running away again. Aoe fears and deathcoils to get rid of pursuers. I like the idea of a discipline priest as the healer though. Glyphed holy nova would heal your guys and damage anyone in range and it can apply a shield of some sort. The priest can pain suppress themselves too.
(and isnt there reflective shielding throwing damage back).

Also if you have one of the afflocks specced into curse of exhaustion you could make it harder for a lot of classes to catch up to you.

Then again the disc priest could also be turned into a shadow priest if the healing isn't working out.