At last for me are the healer separated enough.
White mage is a pure healer, who take care, after the dmg happens.
Scholar is barrier healer, who prefent dmg (is the healer, i like the most).
Sage is the most offensive one and the mixed child of the first 2. He allows you to adjust you, based on the co-healer (barrier focus, if no scholar is there, and pure healer, if a other barrier healer is there).
Astro is the only problem child, who is a buff support healer. But had bigger changes over time (didnt tested the current one, he was the one, where i had the biggest problem in DT).
They are similary, because the heal spells are the ones, everyone needs and make sense (Single heal, stronger heal spells with more cost, HoT, group healing, Esuna, and maybe barriers for tank buster/big aoe). That reduce the room for individuality.
But yeah, they stated to want to improve individuality to the next expansion. But personaly have i my doubts to. For me, are the classes differently enough.