Well, in Cataclysm & Mists of Pandaria, only 1 tank at a time could do high DPS, with the Vengeance mechanic.
But a bunch of DPS players complained about it on the forums, so it was removed in Warlords. Then the same DPS players came and complained about a ‘tank shortage’. Hrm…
At the high end (high keys, Cutting Edge raiding) just about every tank is trying to maximize their DPS.