In Castrum Meridianum, Praetorium, and the two Crystal Towers odds are you will be dealing with 5 to 8 trash mobs at a time instead of just 3 or less. Each of these Trash Mobs will further have more Hit Points than a Standard Mob outside of a Dungeon. Thus using AoE's is generally your best bet for dealing with them, and if you aren't using AoE's, then you will probably be asked to as it is the most optimal path to defeating them.
Similarly in Speed Runs of Dungeons such as Brayflox (Hard) you will be expected to use AoE's so that your Tank isn't overwhelmed with trash mobs, and so that your Healer doesn't have to spam Cure III's and Medica II's too heavily.
