Tank Mitigation vs Tank DPS.

[ReiMakoto]

4% of defence mitigation bonus will have more impact on the team than 1-2% more damage on a tank that does anywhere from 20 to 35% of total team damage.
Just talking here about dungeons.
4% on mitigation alone, and another 4% on healing bonus which is not confirmed yet, but may be a bonus for all incoming heals not only self healing.
This means a tank with tenacity melds will be 8% more tankier than a tank with any other melds.

It is significant amount.

Dungeons hit like wet noodles if you're using your cooldowns correctly, also the healing thing is as you said uncomfired so you don't count that. EX roulletes are already possible to mass pull with minimal gcd healing, the 4% extra is frankly unecessary and a drop in the pond.