To me it does get frustrating, but only because I'm used to not marking. I sometimes mark it if people request it, but more often than not DPS usually knows to attack what I'm mainly attacking. However, I also do know how to maintain crowd control at level 47+ (could be earlier but can't remember), so DPS should be able to attack whatever they choose in the end.

However, if people don't know the dungeon (and I ask them this first), I will mark the targets from most important to least important to take down. Marking, for me, is all situational based.