Noct AST, just as SCH or Disc/Drood in WoW requires encounter knowledge, because you have to bubble up before the hit lands.
Personally I prefer reactive healing (Diurnal/WHM) to preemptive healing (Nocturnal/SCH).
Hitting a button and seeing all them bars refill is more fun than hitting the button and preventing them from emptying quite as much. :<
BTW: Noct AST is useless when you have a SCH in your raid. Bubbles conflict. >.<