I have a habit of doing that too and most of the time it's completely fine. A tank should be aoeing a pack with flash or other class ability they have before they start their combos anyway to avoid stay mobs. Some tanks dont know how to approach a group of mobs properly. If the tank is a second too slow and your regen critical strikes it can generate a fair amount of enmity and position the mob away from the tank. With a 2.5 second global cooldown (the delay between next action) a stray mob can do quite a bit of damage until the tank can regain control.
