From a healing standpoint, I would say that pairing AST with WHM will allow AST to have a little more freedom in tactics, since it can do the crazy overheal thing in Diurnal as well as throw around shields in Noct. AST shields are honestly fairly decent, albeit the ST shield not as amazing since it won't double the shield on crit.

I also really like what happens when an AST goes bananas on groups of trash with Gravity and you have foes up. It's pretty nice for a ranged AoE on a healer.