The difference is both SMN and BLM have ONE main job, hit stuff as hard as possible.

Healers (those optimal ones anyway), have TWO jobs, make sure people don't die and also hit stuff as hard as possible.

Having the ability to do both jobs at the same time is Scholar's main factor in being mandatatory.

Having TWO oGCD AoE heals (Indom and WD) is pretty weird as well for a class that "is not suppose to handle AoE healing well".