What if...
1. AST heals more powerful heals than WHM?
2. All content is created to be viable with any combination of healers?
3. PLD/BRD/AST/SCH mitigation makes the heals WHM brings laughably unnecessary?
4. SCH's tether makes it a very strong MP free ST Heal?
5. AST has the strongest aoe heal?

That said, I can see a WHM/AST being meta, with WHM being the DPS off healer.