As I've mentioned in another thread, dps is the only thing in an mmo that doesn't have a ceiling. You can overheal. You can over mitigate. More dps is always useful. It's the same issue with "support" dps-- you don't always need those support abilities, but can always use more damage. By the same token, people don't want homogenous classes and you can't have Warriors unable to tank things just because they have more damage.

Giving more damage to the other tanks is touchy because then the start encroaching into dps territory and we all know how much MCH/BRD were complaining about being beat by Warriors before the buffs to GB/WM.

I'm not exactly sure how you can fix this problem or if there really is a solution without making all tanks carbon copies of each other.