I ran Leveling today as a WHM.
Got Sohm Al (Normal).
Tank proceeds to run straight ahead; I barely got Protect out before he moved out of range. He throws his shield and keeps on going, pulling not two, but THREE (IIRC) groups, going clear up to where those poison plants are.
Thing is..... one of the DPS were AFK.
I'm spamming Cure IIs on him left-and-right like crazy, which means I am doing zero DPS and I even had to blow Largesse, Benediction, Asylum, and Lucid Dreaming and nobody died but we came out of that with me having 30% MP left. The fight just went on and on and on and on because we were a DPS short.
We vote-kicked the AFK DPS and got a replacement, and the rest of the run went fairly smooth, and thankfully the tank learned his lesson and only pulled 2 groups thereafter. I had to use more Benedictions and Lucid Dreams than I normally do and I wasn't really enjoying it much because it left me with zero DPS time from chain-casting the cures.
Turns out, at the last boss, the tank rolled Need on the sword... apparently he was still leveling and didn't overgear the instance, but yet he thought pulling 3 groups from the start was a good idea. Ya.
Anyways, I say this because I really can't understand healers letting tanks almost die. You have such a low margin of error, especially if the tank isn't level/item sync'd for the instance. Whenever I started casting Cure II when he was at 50%, it landed when he was but 2 seconds from death. It was just a chain of Cure2 Cure2 Cure2 Cure2 Cure2 and I didn't even have enough GCD time to throw a Regen let alone any DPS.
And there are healers who don't even start casting until 30%? Oi. That's asking for trouble. There's a reason why some tanks panic when their healers let them get that low. One bit of lag, or having to move out of an AoE, etc... and the tank is gonna be dead in that sort of situation.
EDIT: And the tank was indeed using his defensive cooldowns. Dunno if optimally, as I was more watching my Cast Timer and spamming the Cure2 button, but I do recall seeing Conva, Rampart, and on that first pull he even blew his Hallowed Ground.