AoE, that's why. We wouldn't have even got to atttempt Akil'zon without either a mage or warlock in the raid and later on there is at least one boss where we'd be totally screwed without AoE.
Rogues and hunters will out DPS you on single targets, you have AoE. A bit less DPS on a single target is not often raid breaking but no AoE when you need it is.
Plus decursing, decursing mages make healers breathe easier