I gave up on Nocturnal after a few runs of Dusk Vigil. The difference in effective healing potency between the two stances is quite significant. Less potency means more heals, which means more MP spent. Given that MP is already a bit tight for AST (especially when you're trying to contribute some DPS), I'd much rather have more efficient heals. Obviously, if you're with another AST, one of you should use Nocturnal since our regens don't stack (but they do stack with WHM, which also stack with each other, for some reason). Otherwise, Diurnal is by far the stronger choice.

SCH was my main for most of 2.0, so I honestly would have really preferred to focus on Nocturnal. But it's just generally not worth it.