Just for the sake of context, SCH got buffed in that situation shortly after AST was released because AST (Noct) was straight up better than SCH in every single way.
Their Shield spell was:
1) Instant Cast
2) Higher Potency
3) Lower Mana Cost
AST as a job did nothing but cause balancing issues since it was implemented.
Either it was a jack of all trades / master of none who had buffs to compensate for it's shortcomings, or it could do everything the two other healers could do just as well or better and had buffs on top of that.
It wasn't until SE basically neutered Cards and healers in general that they simply became carbon copies of each other with minor job flair.