I think that would work for 1, maybe 2 healers, but having all of them focused around buffing is just as homogenous. Ideally they should focus on different playstyles, with AST being the primary support/buff option, and the others can have varying degrees of higher personal output vs party output, i.e. WHM or SGE being far more bursty and powerful with no buffs (WHM being slow and strong similar to BLM, SGE being fast-paced and mobile), while SCH has more sustain power from its DoTs and Chain Strat for additional support.