Quote Originally Posted by Zfz View Post
Actually I simply know which mobs are put to sleep. The sleep sound effect is extremely distinct, mobs get bubbles coming out of their heads when put to sleep, and I think it is obvious even from the targeting arrows.

Part of being able to tank properly is awareness, and I think all tanks should learn to be aware of the whole battlefield all the time. Knowing which mobs are asleep is a good first step.
While I can't argue that mentality, we're talking about failsafes here. Marking a sleeping target so that I know to avoid hitting it for the time being can and does help if it's a busy moment, lots of crap is going down everywhere, and you just don't happen to catch the sound of the sleep spell and you can't see the small sleep bubbles. If you end up in a rhythm of rotating while pulling aggro, it can be easy to miss even for the best of people. Nothing wrong with a mark there as a failsafe.