this is because you start blasting heals on tank the instant they agro a pack
if you're not cc'ing anything, 1 dmg ae from the tank should be enough to keep adds off the healer for the whole fight.
keeping them off dps is another story of course
DPS who aggro strongs and start taking damage = need to heal.
A tank who jumps in and leads with a taunt before an AOE damage ability = healer will rip in exactly 6 seconds if they toss any heals at all, let alone an AoE heal or a HoT.
You make all of your generalizations thinking tanks know how to play their class. By and large, on my server, half the tanks that queue without full groups are DPSers in tank stance without gear and skill, or are in tionese or worse. My healer is in full campaign, my MDPS in half dread guard. I have to remind the tanks *constantly* to guard me as DPS, and if they don't, I apologize to the healer immediately for making them work hard. As for healing .. if I have a bad tank, I can keep them up, but i spend most of my time DPSing because otherwise I'd be meat.
That's why I pretty much exclusively run guild HMFP runs now. There's far, far too much bad.