To go full AOE or not is a decision that needs to be made from the start of the battle, and there are more reasons for me not to go full AOE. One of them is me, and the tank can kill a larger mob while the other DPS and healer do AOE with the support from the tank who almost always does AOE, and in the end, all mobs will be dead at the same time due to the more substantial DPS on the bigger guy and in this scenario could even shorten the fight.
Also, as a red mage, I will never do AOE if there are fewer than three mobs.
I'm not sure if you are DPS, tank, or healer, but you're doing lvl70 content, so you should have some experience and understand this concept.
But in the end it's up to the person in question how he/she wanna play the game.