Yet we all know what is really happening when it's a WHM/SCH paired raid. That hasn't changed at all.
See the threads asking for WHM's 20y HoTs, this one for MND bonus, among other things that ASTs want to raid from the WHM toolkit. Now why is that?
AST is supposed to bring buff utility at the cost of potency, it's not reliable. The combo is kind of like how DRK negates the loss of the highest defensive tank (PLD) with it's higher tank DPS and raid utility skills like Delirium, yet with the most inferior immunity cooldown. Clearly one job is in a better position because it is reliable utility.
Sure, YoshiP did apologize for flattening stoneskin and proshell, but also on the same line said they made up for it by giving WHM more healing power. Sure, SCH can perform the main healing role. Albeit with Indomitability requiring Aetherflow and Emergency Tactics also being a cooldown. Is it optimal? No. But it can work.