I keep reading about how DH, Crit, DET and/or TEN scale etc, but 9 times out of 10 the discussion centers around damage dealt, and the discussion proceeds along the lines of "Tenacity doesn't scale as well as DH, Crit or DET, so it is trash.".

IMHO that's wrong because you are discussing only one third of the impact of Tenacity as if it were the whole. It's not.

If TEN scales at about 200 points = 1% (the best analysis I could find suggests this), then right now I have an almost 6% dmg buff up 100% of the time; in addition (if scaling estimates are right) I have an almost 6% damage mitigation buff with 100% uptime and a similarly scaled healing buff any time I cast Clemency. If Savage gear allows more than one t6 meld and advance melding of t5 materia, then Yoshi's statement about the impact of Tenacity could easily be on point. Perhaps, we could be looking at anything from 1600 to almost 2000 Tenacity with fully tricked out savage gear. 8-10% damage buff, 8-10 % mitigation buff and 8-10% healing buff, all with 100% up time. I don't see how DH, DET or Crit can compete with that.

How the secondaries are evaluated within the game engine, the order in which they are applied and how could magnify that impact still further.

With DET also affecting healing, and Skill Speed impacting the speed with which skills come back as well as the damage of auto attacks, it seems to be that the strongest overall combination of melds would be TEN>DET>SS. That's probably not the strongest straight DPS combination, but the overall impact on tanks would be greatest starting with Tenacity - as it should be. Of course if some players want to continue melding Str or whatever to maximize their DPS, that's fine, but it seems as if players focusing on TEN will be only 2-3 % behind in terms of DPS, and yet have superior passive mitigation and healing capability.

Since we are tanks, I'd think that mitigation would figure more highly in the discussion. Apparently not.