It seems your gripe is mostly with content design then, the reason Tenacity isn't necessary is because content is designed to have certain damage windows, so as long as cooldowns are being used properly you'll never need the extra passive mitigation, which is the reason we are able to go full Crit/DH and still beat all content (i too have cleared TEA with full Crit/DH melds).
I will never claim Tenacity to be better than Crit/DH, since again, player skill will always trump any melds you have, and proper use of CDs is called player skill, however to say Tenacity is undesirable is not right, it might not be desirable to you because you'd rather have the damage output from Crit/DH and rely on your own skill to mitigate damage, however for others it might not be the case, they might choose to play it on the safe side and rely more on damage dealers to do their part, which is completely fine seeing how Tank DPS is for the most part irrelevant at the moment.
Yes, i can only compare raw numbers.
You guys seem to be missing the point of the stat, damage mitigation will always be something you use to "play it safe", to give your group more breathing room, specially something that gives passive mitigation, if you're playing at a skill level that doesn't require it, then it simply isn't necessary.
Back when it was Parry materia it could be called worthless, not only did you lose a lot of damage from not melding anything else, you also didn't get much mitigation, so there was no point whatsoever in getting that materia, however that changed going into Tenacity, now you get guaranteed damage and guaranteed mitigation, which is the entire point of the stat, it's working as intended, if it were even slightly better (again, small percentages), it'd just tower over other stats and then we'd be looking at making other stats better.
So the fact remains, want damage? Meld Crit/DH, want mitigation? Meld Tenacity. Who wants to meld damage? Players capable of relying on player skill for mitigation, who wants the extra mitigation? Newer players or those unsure of their ability to mitigate properly. Not everything is for everyone, but the impact that Tenacity CAN have is absolutely undeniable, an additional ~6% passive mitigation is quite good (again, small percentages make for small changes but that's what we're discussing).