If DPS don't know what to attack first by the time they're 50 something is seriously wrong. Watch the tank, attack their primary target first. Not difficult. For the majority of the dungeons there's an established kill order which tanks will follow. Marking is a crutch for clueless DPS in my experience. I only do it when I notice that DPS are being chumps and attacking targets out of order. Or i'll do stuff like mark the gob that stops the birds spawning in Brayflox. English players struggle with hitting correct targets a lot more than JP players, especially in Duty Roulette.

Also, dotting multiple trash mobs is pointless.