From my perspective it seems reasonably balanced. All three healers can take LA. WHM has Assize and Thin Air, SCH has Aetherflow, AST has Ewer. Ewer is the least reliable due to randomness, but its free and can be sleeve drawn or even spread, if you're REALLY struggling. And Thin Air, while fantastic, isn't without drawbacks - movement issues after using it come to mind, plus the cooldown.