As a healer I have noted that a few tanks were not switching targets to keep generating enmity with all the mobs, in fact a number were basically pulling and then stuck on one... 1 cure 1 later and I had aggro, luckily most of the tanks responded maturely to my suggestion they switch targets often to ensure all the mobs get a piece of the hate.
Thats why the tank needs to mark the DPS targets, you cant simply target the tanks target as the tank should be switching to other mobs to keep the aggro on all the targets.

Of course you do get healers that will just spam heals and expect the rest of the group to defend them, but at the same time healing is a AoE aggro generator and on random tanks you cant let the HP dip too low , not until you learn the tanks personal ebb and flow of damage taken and healing needed... I have had paper tanks that I can barely keep alive with constant spam and tanks that just dont seem to need healing at all nearly, depends on a lot of factors.