it's not the tank's fault if you OoM from trash pulls. if they're undergeared and they overpull from inexperience, guess what? they DIE. OoM comes from poor DPS, not tanks. the only time it's solely the tank's fault is if they decide tickling the boss is ok when you're at double digit MP. and even then, tunnel vision is a pretty common flaw.
DF isn't "everyone better be perfect" time. DF is a test of patience, and if you fail that test, your anguished soul will be ground up and thrown into the pot of bitterness and anguish that they use to create the next raid tier. your tears will be used to sustain our SE overlords as they laugh at your despair.
if your tank is REALLY that bad, you last...what? 5 minutes? 15 minutes? or you adapt. its that simple. no. it REALLY is that simple. if EVERY single run is an OoM horror story, there's only 1 common denominator
...either that or ease off on the freaking holy spam if your tank needs that much attention. like... could you use 1 or 2 less? and actually save the MP to heal them? because if not, its your own dang fault you went OoM
