TL;DR: TEN is not a bad stat, it's just the worst one on tanks. Good on gear, awful to meld, there's no point. It's the same logic behind Slaying Accessories on Tanks from ARR/HW. If you don't need the defense, do not gimp yourself and your party, just mitigate properly in the scripted timeline. Healers don't need you to hold their hand.
Unfortunately, you are operating under the assumption that Damage, Healing, and Damage Taken are on a level playing field. In fact, you put damage below those other planes, when two tanks (or two optimized healers) doing proper rotations do more damage combined than any single DPS in the game, which matters a whole lot when you're pushing enrages in real content. It absolutely 100% matters how much damage every job in the game does, regardless of role. Not the actual number itself, just the act of doing damage is the only real metric the game has as a variable the player can influence, as everything else is scripted, by the books mitigation. We play a damage focused game, this is due to design, nothing hits hard enough or randomly enough for that ever to not be the case without significant combat design reworks.
With that being said, TEN is not a bad stat. I'm rolling with 1833 TEN right now. Here's a link to the stat tiers this game uses.
https://saltedxiv.com/stat-tiers
Looks like a 1.044% modifier on those relevant tank stats. Were I to pump that TEN stat to max 5.4 value recorded at 4835, which you can't without max TEN only gear + melds if it's even possible at all, you grab a modifier of 1.135. In very very dumbed down terms in order to make a point, a 40K Cure II looks like this, omitting DET atm.
No melds: 41,760 HP restored
FULL TEN CHAD: 45,400 HP restored
And that's when the game is being consistent, which it isn't at all because of high/low damage rolls on attacks equaling variable damage, high/low rolls on healing equaling variable HP restore amounts. You can use a similar formula for mitigation taken and damage done. The issue with this is that when enemies are hitting you for like 40K damage auto attacks, and every tankbuster this tier does like 200K unmitigated damage at a MINIMUM (Umbra Smash and Formless Judgement easily go 800K+), who actually cares about getting single-digit percent increases on defensive attributes? You know what isn't variable? Cooldowns.
In fact, on all significant damage taken, you either invuln it, where TEN is effectively a dead stat on every tank at that specific moment, or you super cooldown with everything in your kit, where you would die or survive based on mechanics of the attack, where TEN doesn't matter at all, or WHICH ACTUAL HEALS are used on you, not the actual value of the heals used themselves ie: 41K vs 45K wouldn't be the difference between life and death, but Essential X2 + Synastry Benefic II + Excog would save you regardless of your TEN stat, as even with the lowest possible rolls on every heal, it would still be enough to keep you alive. It's particularly bad when you realize you have tanks walking around with 210K+ HP, you really think that TEN is going to save you from the fact you just didn't mitigate properly on the incoming damage? You are significantly more likely to come across a situation where you hit a .4 or lower enrage cast than be saved by TEN. And a tank who doesn't meld correctly would be doing objectively less damage because CRIT, DH, and DET are objectively better stats to increase damage, to the point where if you meld TEN ONLY at the expense of other, better substats, I'd argue that you're griefing your team. Just because a stat is DESIGNED for that role, DOESN'T mean that it's good, whether that be due to SE's incompetence for designing stats (LOOKING AT YOU EXPONENTIAL CRITICAL HIT GROWTH AT THE EXPENSE OF EVERY OTHER STAT LATE EXPANSION FOR THE THIRD TIME), or for the logistics of the stat just not making sense on that particular job ie: DNC/MCH with Skill Speed, RDM with Spell Speed, WAR with Direct Hit, literally any healer post progression content and gearing with Piety. Offensive stats, particularly Crit and Direct Hit are never dead stats. Ever. That's why they get melded over TEN, and they just do more "offensive" than DET (except on WAR), so that's the priority. You could rally behind "Why are tanks allowed to meld DH?" to which I would say "Yeah, why ARE tanks allowed to meld DH?"
Since I can't go a single post without talking about DRK, let's have a sample scenario of a DRK melding TEN for "saving healer GCDs" or whatever. When Healer oGCDs are so insanely powerful right now, you hardly need to GCD heal the tank ever. Congratulations, because you melded TEN, and didn't meld SkS, not even CRIT or DH, SKILL SPEED, you now have a GCD of 2.47 (something that actually happens on crafted gear in 5.4). Because of that, you can no longer get 5 hits in Blood Weapon. You literally cannot do it. So what does that mean? Each minute, you lose out on 600 MP, on top of just generally regening mana slightly slower. After five minutes, you've failed to generate 3K MP where other DRK's have that gain over you, which equates to one TBN. The value of that TBN is 25% of your HP value, so let's pick a flat number of 200K. You've essentially added 50K HP of burden onto the healers for no reason every five minutes. Saying it like that, it's actually even worse, because bosses are not constantly attacking, they have casts, they have phase transitions, and all that downtime stuff. The percentage of time taking damage vs not taking damage is not as wide as people think, particularly in something like E9S, where it seems half the fight is casts or tankbusters, not autoattacks. These assumptions also place you as the permanent Main tank, which shouldn't be true in any fight. What is TEN doing for you as the Off-Tank? Nothing. Nothing at all, compared to Critical Hit, Direct Hit, or Det. Point of this being, if you want your healers to damage more, use mitigation in creative ways, like doubling up on auto attacks + busters, better reprisal timing, actually using Arms Length in dungeons, and other methods than crippling your own Crit rate and damage.
Addendum: I didn't even realize this when I wrote this, but if you're a WAR and you meld TEN, that's even more ridiculous, because Criticals get boosted in damage and rate with their melds, and you heal a LOT more off, and thus "mitigate more damage" on a Critical Fell Cleave with Nascent Flash than if you just melded TEN.
To summarize, if you melded TEN, and you survived at 1% HP, nice! Not only did your healers not heal you, you also probably didn't mitigate auto attacks before/after you invulned the tankbuster, got a low roll on the damage that actually dropped you to that amount of HP, and you're going to die to the very next thing that breathes on you.
If you melded good stats, congratulations. You just saved your team a couple thousand overall raid DPS that they needed to kill the boss on the enrage cast, you've given them a higher margin of error for making the check in general with proper play, and you can move on with your life.
And this is in savage, where stuff actually hurts. Casual content isn't even worth a paragraph, people do dungeons/NM raids with zero CDs all the time, it's irrelevant to this point.
Unless you're progging Week 1/Ultimate where that can be a discussion with your healers and cotank for comfort, do not meld TEN at the expense of other stats, it's pointless.