I do the same thing when I heal, if DPS pull threat or run ahead, I don't let them die; instead it's a fun challenge in an otherwise boring/monotonous instance.
My GF, who heals mainly, but tanks from time to time, is the exact opposite. Will let ANYONE die who does stupid shit, and then mock them after. I get that stance, to a degree, but in my opinion, it takes too much of my time to let them die, explain that they're bad, and rez them. Plus, they likely won't learn anyway...may as well at least be efficient.
ON TOPIC - Threat at 90 on trash is a bit shit at the moment, compared to a DK and esp to a monk. Keg smash/dizzying haze is SO OP on threat, so don't feel bad losing aggro to a BrM. My co-tank likes to aggro ALL the things, but since the mobs in HoF stun/silence, he usually gets insta-gibbed and I pick up the pieces. From then on, we share the load :P Boss threat is cake, and if you really want to make trash easy, just chain pull to keep that vengeance rolling.