In my experience, a lot of tanks who refuse to mark are the same tanks that can't keep aggro on multiple mobs. Don't complain about what mob(s) your group chooses to DPS if you actively take no part in group organization. I especially like it when the tank tells DPS to just watch the enemy health bars. Again, my experience. I don't personally care if a tank marks or not as long as they aren't a jerk about the possible consequences.