Then they should lower PLD DPS and give them a more supportive role not just defensively, but also give them DPS buffs for their group. Then their personal DPS is lower, but they contribute more towards the group. If it's just defensively you'll get HW all over again. The highest DPS tank gets invited, seeing they can all survive the tankbusters, and PLD will be left out.
Seeing the above will never happen, for the simple reason a PLD is more a bastion then anything else, having simular DPS as the other tanks is the easiest way to get PLD in the running again. Yes, I said simular, not ahead.