I have been told by many lvl 75 JP and a few high lvl NA players that it is against the rules, and i have witnessed a GM called on someone who was doing it. Even if its not against the rules its wrong, I would only see it acceptable if the area you are farming in is empty. AOE spells have uses in high lvl Magic bursts or for finishing off multiple enemies if your puller gets link and the party decides to take on both. I don't think their purpose was to link 30 mobs and kill them for gil.