Why would you want a warrior to tank? Sure, warriors can tank fine. But if you could have a DK or a Pally, there is no way you would take a warrior instead. Pally's can self-heal, AoE, and bring useful buffs to the table. DKs can self-heal a little, lots of self-survival cooldowns and have great AoE.
Again, warriors are fine at DPS. But that's it. Fine. Why bring a warrior when you can bring in classes that do the same damage but also bring buffs or unique and useful skills. What do we have that's useful? 4