Having played many MMOs both PvE and PvP I have often found the issue of tanks being a bit less useful than Healers/DPS in PvP. One system which did a great job of making tanks useful in PvP was SWTOR. For everyone who didn't play it: Each tank has a guard which he can place (and switch) on a group member. If the guarded players stands within a certain range of the tank, 50% of the damage is redirected to the tank as well as some damage reduction. Also taunt does decrease the damage a target deals when hitting a player other than the tank while under the taunt effect.
This does not mean that this system should be copied but it is to showcase that there are great ways to make tanks viable in PvP aside from Crowd Control and just absorbing a ton of damage. Because in most MMOs this alone will not make tanks a used specc in PvP. Do you guys know other great usages of taunt or other tank mechanics for PvP? I only played WoW, GW2, SWTOR and FF14 so there are surely more great options