Mixed teams take more work, that's for sure - just setting up your macros is a major task - but the benefits are pretty good - buffs, portals, free food and water, soulstones, fears, crowd control, ressing, DOTs, etc.

Destro and Demon locks are great for pure DPS.
Mages are great for food, slows, AOE, portals, and buffs.
Druids are great for ... everything. Heals, DPS, offtank, you name it.