If you look back to T14 and T15, the use of ES was mostly when HP and LH was bad, not when ES was good. The fights that you used ES for was when you would not really get any decent healing out of HP or LH, or when the content was on farm for rankings since the heals was not needed. Going into T16, tanks are a much smaller part of the damage done which makes ES less attractive. The only real reason to use it is if you want to rank on farm or if you really dont get any effective healing done with HP/LH. There has been one exception to this during this tier, and that is siegecrafter, mainly heroic, on which ES is the goto talent. Siegecrafter N or H is the only fight that I would actively recommend ES. For all other fights you should really find use for HP/LH, but there are several fights where you can use ES if you feel like the healing is fine, fights like Sha of Pride, Nazgrim and Malkorok and probably a few more you can use ES if you really dont need the healing, but likely nothing you would do during progress.
I am wondering that too
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I do not feel like the health pool is an issue on most fights. I am barely breaking 800k health with stamina flask and stamina buff, and my armor is fairly low as is my haste (44%). Yet I have no problem in heroic content. I never felt squishy really, but felt like I needed more gear still so I could stop using stamina flask.
With 545 item level that I have currently now, everything felt easily manageable, but I still had to pay attention. Granted my tanking experiece so far is 14/14N with thok and siegecrafter on heroic, I was not part of the farm last week, going in today and clearing the other heroics so I will see if my opinion changes then.