The only time not to mark is when everything is dying to AoE, because it will all die at the same time anyway. Not marking when everyone is single target makes you need more healing because split DPS leaves things alive longer. People who don't mark are just very, very lazy for the most part. You don't even have to slow down to mark a 1 on something, do it as you move into range. I don't bother with 2 3 4 5, etc, but at least put a 1 up on what should be killed first. Yep, I'm lazy, too.