And since the healers are pretty well consistently healing at that point, they'll aggro them in no time, especially if it's a sage/sorc dropping their AoE. Consider that the tanks have no threat on any of the mobs to start aside from what they spawn with, and the healers are generating threat pretty well constantly..
Not necessarily. Trenchcutter threat generation is nonstandard, at least while they're under the shield (in Ni. I would guess in HM as well, but we never used a strat in HM that would have caused me to notice, so that's just an assumption). I've seen--several times, while we learning the fight--healer aggro pull a warrior out of a pack of cutters while the cutters continued on to the tank. Beyond that, my group's approach to the cutters goes like this:
Tank guards other tank. Guarding tank and dps stand behind the walker. Guarded tank and healers move in front of the walker. Guarded tank and healers peel the warriors as they spawn. Cutters beeline to guarding tank.
And we have never seen cutters change target to the healers before reaching the group behind the walker.