I always mark targets as a tank unless I'm feeling spectacularly lazy or I'm speedrunning with two black mages and a white mage that are going to blow everything to smithereens with mass-AoE/Holy spamming. I tend to flip back and forth between all the mobs as I tank to make sure I keep hate up on all of them, so it can sometimes be difficult for the DPS to determine which one I want to kill in what order unless I mark'em up.
As for gremlins - I kill them first because the little bastards deserve it for callin' me an idiot. >:C
As for gremlins - I kill them first because the little bastards deserve it for callin' me an idiot. >:C