I had a mentor tank last night on my 49 Machinist in Aurum Vale (Leveling Roulette). I died twice, since he was in such a hurry he didn't bother to hold the mobs properly, so the 'not well-held' mobs killed me twice. I did nothing special, just AoE like other people did. So, yeah, some mentors aren't so wonderful. Once again, it's all fine and good to pull wall to wall...but only if you can handle it right. If I have to run back because you tried to pull too much, once I'll forgive, but twice, you should pull a bit less.
If I die as a DPS or healer because the tank either pulled too much, or couldn't hold the mobs they pulled, they need to pull less. Mentor or not.
EDIT: Basically, if I have to run back because the tank pulled more than they could handle, any 'time savings' are lost. Pull less, that you can handle.