Lets say there is an NM.
He has 6 minions. Three of Type A and three of Type B
They are all gathered together
You have ~6 people with voke/taunt
You want each of these people to hold hate on separate minions
It becomes hard to plan and explain this tactic if you can mark the minions to distinguish them from one another
I think one solution is allowing free roaming mobs (specifically groups of mobs) to be marked only if they are claimed. The markers are most beneficial when assigning kill order to a group.