Tanks can't keep aggro.

[Twilite]

Better Tanks than me have given me some sound advice. "If they want to keep taking the hate from you let them keep it."

You say it like healers are trying to take aggro intentionally. You're a tank. You know better. Heal spells are silent creepers in enmity. If you only worry about the DPS attacking your target, the healer is going to be staring at red triangles on every fight, for that reason alone is why AOE threat is necessary.

Also I'll be very bold and say those aren't better tanks than you telling you that. Those are morons.