Dont dispel a tank if he's paralysed, unless you feel you still heal enough.
Dont dispel a single poison, dispel 1 or 2 maximum only if he have 2-3 poisons (make sure he's high enough before doing the dispels that he survive while you're in GCD)

Myself, I see only tanks that take as much monsters as there is on the map before getting block by a gate or something.
I think, when they see they take like 6-9 monsters and I still take time to dps with Holy, they feel confident and take even more after.

Once a tank took all before the last boss of Sastasha (those last monsters hit hard), I had time to do 3 holy (1 more whould be highlyy risky, 2 more, he was dead)
But when I go as DPS and I see other tanks doing same, 90% of the time, it lead to a wipe (once I think the heal did it), and they dont even have time to Holy, they just cant heal this, whatever the gear they have (even saw healers with better gear then my healer)

I dont understand how they can fail it, it's simple :
Divine seal > Cure2 > regen > the buff for speed while you're in GCD of regen > Cure2 > cure 2> ... > if you see it's goes well > DPS stance > speedcast > 2-3 Holys > return in healing stance > cure 2, cure 2, and dont forget the buff to give mana
Also, if you see his life doesn't decrase too much, try to put some cure1 in beetwin the cure2 to save some mana.

Ok, maybe this is too hard for some ppl, but I feel it's simple, already played things harder then that.