Most instances have "Humanoids" Also if you are Multiboxing properly and keeping a good check on aggro, its entirely possible to Multibox mages. Admiteddly, Warlocks do have the beenfits of soulstones/pets however I'd say the burst capability of mages outweighs this. (especially when reasonably geared)

Whilst warlocks are awesome, personally I'd go for control over damage everytime.