Just a quick bump...
Marking a kill order isn't really done in regular dungeons (from my experience), but it can be a good tool for newbie tanks that don't yet have a tanking stance and may not have the very best gear. If the damage dealers are not on the same page as you (or each other) and targeting different enemies in a trash pack, the mobs die more slowly and it's more difficult to hold hate on the entire group. Simply marking one mob in the pack that you want killed first can be enough to get the DDs to focus on a single target, and by the time it dies you should have enough of a lead on enmity to hold the other two or three mobs in the pack.
As you progress and gain more abilities, the trash in normal dungeons is much easier to tank, but when a BLM in Sastasha rips a mob away from you before you've had a chance to even Flash it, it sure slows things down and makes for a frustrating run.