Lightspeed is amazing and using it for stacking AoE heals or a raise can save you lots of MP.

Also remember Collective Unconscious has a HoT which you can access even in Noct stance, not to mention the sweet shield-bubble that it is.

AST (more than WHM for sure and perhaps more than SCH in some ways) requires a bit more thought about your MP management. Less of the MP reduction/return is frontloaded into parts of your kit you would be using anyways (Assize, Aetherflow) and in my opinion it's more reliant on the AST making the most of their kit so they don't need to Ewer themselves.

That being said on WHM I got an Enhanced Ewer in Susano EX from my cohealer the other night which pretty much saved us from a wipe. Sometimes it's a good idea. Don't be scared to use it.