Read some of the threads in the Tanking Forums. It's not that Warriors are weak or incapable of end game content that is the issue. The issue is that they require more constant, undivided attention from the healers in comparison to the paladin tanks because of the overall lack of mitigation. It takes more mana intensive for a healer to keep a Warrior tank up than a Paladin. Read those threads. Why would a group want to bring a tank that takes more damage and requires more healing over a tank that doesn't?