There is 3 role in the game, healer that their work is heal the team, dps that their work is do damage and tanks that the works of them its damage mitigation, hold and stay live.
Speaking about pvp the tank its like a combination of both (healer and dps) and lose their principal role in the game.
In terms of pve, tanks are like leaders, they lead the party, marking objectives and hold the damage of mobs and bosses helping their team to stay alive and focus in one target.
Why dont aim tank to that type of gameplay in pvp? i mean, it will very very good if tanks are oriented in keep your team alive with some defensives and damage mitigation, if they can bring some dmg buff (like warrior in the past reducting armor) and order the party attack that target, if they can hold on a point in bg until their team arrives, if they can help with cc tools to control the battle, if they can help their caster to have a better time to cast.
It will be so epic if you are going to die and a Prot Warrior charges at you and put his shield and save you from death.
But now they are oriented in do massive aoe dmg and that its the work of dps, if the tank cant do that, its complety useless.