I wonder if the group makeup (including level) affects the tank merc behavior. I've been fooling around on a level 50s rogue alt with a tank mercenary, so molo. The tank merc is set to Auto Assist and Agressive stance. The only group role I used was Main Tank. However, the following behavior didn't depend on whether or not the tank role was set. When I aggro multiple mobs or get adds when fighting one mob, the tank merc won't switch targets to take aggro off me unless I target one of those mobs. Once the merc gains enough aggro on that mob, I have to switch targets again so the merc will peel aggro off the next mob. It doesn't matter if I turn off attack so that I'm not building aggro on the mob the merc is attacking. Even though my aggro percent is low, I can imagine the mercenary might not be happy about the numerical difference in aggro.