Imagine an impenetrable fortress tank with no attack power in PvP. You would probably just ignore them and kill off the healers first and then the glass cannon dps. Tanks soak damage for others, not for themselves. It doesn't make sense to expect a tank to get attacked in place of someone else unless they're a genuine threat.
This should not be a trade off. Tanks should get both offensively and defensively stronger with better gear.
