WHM getting progressively more unnecessary in content?

[Yeol]

I didn't say that slightly stronger healing potency is making up for their lack of utility.

Try going double WHM and let them decide who will be MH and see how much DPs the other one can put. You'll be amazed.
But like I said, sadly WHM cannot do both at the same time due to their tight MP. If they had better way to recover MP you'll see them spamming holy and helping with killing ads rather than focusing all the time on healing because if they do anything else they will cry for MP.

SCH and AST have no problem healing + dpsing and they have utility on top of that.
While WHM can heal but they will have trouble dpsing due to high MP cost. The least they could do is give them better way to recover MP and let them DPs since they have no utility.