But why would you IR in defiance unless you needed to fully heal yourself from 1000 HP or something?
If you already have a high threat lead on the party, any extra threat generated is a wasted resource just like overcapping your job's gauge, or overcapping MP on a job like DRK, or casting heals when no one needs healing. And the only time people rip off me (I primarily tank, so I can't pull examples from DF tanks) is if either: I mess up (you know, people make mistakes), try something different that doesn't work out, or they don't use any instances of diversion or lucid or tactician. Players mess up frequently in this game, just because it's easier to notice a tank messing up doesn't excuse DPS players who don't know how to do more damage output than the WHM in the party, but we're not talking about how DPS jobs are too hard and need to be streamlined so even the lowest common denominator can actually do decent damage (and is a good example of why tank damage can be important to succeeding on a fight that has an enrage, since you aren't beating it in tank stance the entire time + DPS making their own mistakes they don't notice).
(And even here, I've had DRKs and PLDs camp their tank stance and do threat combos, but still end up ripping off them or getting dangerously close to doing so as... just being a Warrior in Deliverance, because that +3 to 4k extra damage I'm doing over them adds up in terms of threat)
IR itself in Deliverance generates a lot of threat: it's a massive amount of damage in a short period of time that'll secure a hefty lead after the pull with the Equilibrium overheal threat on top of it.
Doing something completely suboptimally doesn't help anyone. If anything, doing things suboptimally creates a higher chance of losing on a fight that has an enrage, just because you're busy generating tons of threat you don't need anymore, at the cost of damage output that could've killed the boss.
But again, just because players don't know how to operate a job of a particular role is a flimsy and weak argument to use when trying to justify taking a dump on the role.
And the way stances are now make tanking feel more dyanmic and fluid, especially since you can use it to fix errors, to potentially save runs, or help your healers in a high stress moment, and with WAR you can also just... use Defiance with Unchained any time you're planning to use Thrill with an Upheaval coming up for an overall gain over the course of four to five gcd's. Which does tend to line up with high damage moments in fights.
If it's just pure-1-stance per fight, you lose that interactivity.