Speaking as a tank, I don't have a problem with some kind of tank training instance but they also need one to teach DPS to attack the marked targets and attack them in the proper order. The number of times I have lost aggro on a mob because a DPS decides to unleash on mob number 3 instead of mob number 1 is ridiculous. Fortunately I pull the aggro back almost immediately. There is a reason I place a big floating number above the mob's head.