Diurnal AST is very Competitive. Nocturnal not so much. They have a disadvantage DPSing from the non-subsidized attack Magic costs. MP is much more of an issue unless you can consistently draw an Ewer. They do however have a 5% speed boost for attacking and access to Lightspeed if they do need to front load damage and move. Spreading or Extending a Balance really is where an AST's DPS boost to the party shines. No they won't deal the damage themselves, but without them the massive dps spike couldn't happen. Healing wise, they do not lack throughput. They do not have the immense reactive potential of a Scholar's Lustrate, but they have something similar to a WHM's brute force/preparation healing. Synanstry is the most powerful single target heal buff in game and it is a rescue button in itself. Time Dilation can make you feel very in control extending the time of 3 HoTs and a Bole on a tank by 15 seconds, each HoT ticking for almost 3k... (Thats like spamming Benific at 400 potency @.@)

Diurnal AST is a very nice spot right now. Nocturnal is very very situational and even playable but not nearly as good as Diurnal. AST is on equal footing on most content as of now. (Fixing Nocturnal wouldn't unbalance this equal footing, it would just give AST a second option.)