It has become a staple and even streamlined into MMOs these days and it is one of the things I expect to see in every MMO, the ability to mark the mobs we want to put under CC etc. In a dungeon or even SP grind parties the ability to mark mobs ahead of time will take away a lot of the confusion of pulling multiple mobs and keeping them CC, also allows party members to specify a class or certain player by type of marker for CC or attack.