WHM getting progressively more unnecessary in content?

[RitzNBitz]

WHM makes an excellent Co-Healer to a Noct AST. They can throw regens while doing a substantial amount of DPS out while the noct AST handles the rest. Its just a different type of playstyle that we aren't used to with WHM/SCH. WHM fills in where the Noct AST cannot, not only with regens but with emergency heals that are all OGCD allowing them to be used in between DPS'ing without worrying about GCD's.

In fact i'de beckon this combo is alot stronger then diurnal/sch. Diurnal feels like a poor mans WHM sometimes. And DPS wise on par or stronger then SCH/WHM due to cards.

[Accelerating]

-snip-

Pretty much the other way around for me: WHM feels like a poor mans Diurnal AST: While AST provides the necessary healing, he also brings mitigation and powerful buffs. Also I don't really see WHM as an offhealer, because he is too much limited by his manapool.
Furthermore I don't really understand what you mean by "emergency heals that are all OGCD". Benediction shouldn't be used as an emergency heal, but as a planned healingburst after a tankbuster (combined with HG/LD). Tetra should be used as often as possible to maximize mana efficiency.
Not saying WHM/AST or WHM/SCH is bad, AST/SCH is just better.

[Fernosaur]

In fact i'de beckon this combo is alot stronger then diurnal/sch. Diurnal feels like a poor mans WHM sometimes. And DPS wise on par or stronger then SCH/WHM due to cards.

The problem with NoctAST+WHM vs DiAST+SCH is the lack of stacked regens, and as such, CS uptime will be less for whoever is taking care of the MT. SCH+DiAST can very easily leave AspB + Eos handle most of the MT damage save for tank busters, and big hits on the party will always force the WHM to put up Medica II, which not only takes casting time and less CS uptime, but also big chunks of MP that, in your example, would be better spent DPSing. Whispering Dawn takes care of all that and completely ignores these setbacks.

I'm not saying WHM+NoctAST is bad, I actually think they can work pretty well and also agree that they can cooperate very well in having them both DPS, but that pairing is still inferior to DiAST+SCH by a considerble margin.