Tenacity

[Seraphor]

AND then it'd only be worth it if that GCD of Healer damage was MORE than you lost in order to stack Tenacity as opposed to Crit/DH/Det.

That's the crux of the matter, as opposed to "Tanks just wanna be DPS". It's simply a case of actually getting something out of your stats. The only thing that can be obtained from stats is Damage and in the case of Tenacity defence. But unless that defence is either necessary or meaningful (Either, it enables you to survive, or it provides Healers with a benefit i.e. The ability to use less MP intensive healing to have better MP economy) then Damage is the only real choice.

And unless everyone is hitting every single GCD perfectly, at peak 99 percentile performance, then that one more or less GCD probably isn't having any meaningful effect either.