Tank Mitigation vs Tank DPS.

[Nedkel]

Yes. I said that.
Every 700000 to 850000 pre-mitigation damage taken will save you exactly 1 heal.
Post mitigation, this will save you one heal every 17 heals used. At 700 potency.
Thats 700 potency you dont have to bother with every 11200 potency you're healing.
Best case scenario (for tenacity), thats one less heal you have to cast every 42 seconds.

And it does make difference, could affect the whole party performance with a healer being able to dish out one dps move. Its like a healer having additional extra 1400 spell speed reducing his gcd by 0.12 sec.
This is enough to make up for that 1-2% damage difference on a tank.
The thing becomes even more obvious in dungeons, where there is a constant source of high damage hitting a tank.

I'm actually assuming you will never be overhealing. If you ARE overhealing, tenacity becomes EVEN WORSE, as any damage taken between the time you are at full health and full health that would not have killed you is mitigation that could have instead been outgoing damage. I'm giving you the absolute BEST case for tenacity to show you how bad tenacity really is.
I'm assuming a healer is going to use their healing spells for heals. I'm assuming any heal that also does damage is going to be used for damage.
I'm assuming a healer is not using cure1 or the like to heal.

I think its a fair trade off.
DH is another RNG added to the equation, where tenacity is more of a team dependent stat which could be but dont have to be random.
Tenacity does bring some indirect (hehe) benefits to your tanking, comes with a little drawback and gives something else.

You may wonder why they havent changed this stat for 2 years and will probably not be changed in next 2. Devs clearly has some reasons for doing it.