WHM is generally touted as the pure healer, but when your party doesn't need healing, thats when it becomes limited on what they can do, just DPS pretty much. AST (and SCH) bring a multitude of utility to any party composition, hence why they are part of the meta in end game content.

Also AST can extend Lucid Dreaming and Lightspeed timers using Celestial Opposition. Of course you might not use the extender just for those actions, but those extra seconds can sometimes really make a difference.