AA is like the best skill in guild arena (slightly less so in battle arena)
One AA proc usually does more overall damage than any other skill proc.
In a guild arena situation where you fight the same opponent over and over, it really works in your favor because they will need to spend that AP to heal.
In battle arena it's more situational... Since you're only going to fight each person once, you can't adjust anyway. Maybe they'll have a lot of the same ranged units sitting in a row, if not it might be better to use non-AA to focus fire... Either way, there's no method to predict but AA isn't actually any worse than the other skills, so it's personal preference.
In Colosseum they're like.... dependent on your overall power x_x) They're really good in the lower ranks, AA proc just does some % more damage, a 5*AA can hit up to 6* level damage, so it's quite good. Problem is once you move pass the 4*-5* teams...units are strong enough to win before anyone gets a second hit... so it becomes a contest of whose units get their attack out first... And since AA units tend to be C - D speed, they become useless along with every mage and missile.