I think part of it is that warriors only have 2 routes open to them: dps (whether pvp or pve) and tanking, with no overlap in between.  If you do one, you *cannot* do the other effectively.  That being said, it's been my experience that in raids except for 25 mans, dps warriors aren't needed because the other classes will have much more useful abilties (sheep, sap, totems etc) or be unneccesary due to boss requirements (like Maiden's melee dps limit).  This tends to make most end-game warriors protection specced, and feel entitled to protection-oriented plate drops, which leads to clashes with  protection paladins should they be in the same group.  Personally I prefer a MT/OT warrior/druid combo to avoid those issues.

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