If you have the opportunity to play a tank so well that you literally mitigate/avoid enough damage to the point where you don't need any healing, it would be a far better testimony of your tanking skill than any uber DPS number you could achieve.
Yesterday, I ended with a PLD main tank that only spammed RoH throughout all of A11...it was really sad. But, now, let's imagine something. His WS rotation was extremely bad, but we still won the fight. Do you think he learned anything from that ?
Now, if the WS rotation was part of your tanking capabilities (i.e, generating enough enmity and mitigating damage), he would have died...and we would probably have lost.
Yes, it's a pain to lose a fight, but when it comes to the playerbase, it's better to lose one fight because the MT tank can't be carried that letting bad tanks clear even that kind of content.