WHM is a tricky creature. There's the "here's what they could do with it" angle, and the "here's what Square Enix has been really stubborn about" aspect too. WHM has several design....directions? "Identities"? Aspects which seem to be part of Square's design document on the job which they haven't outright stated but can be inferred from precedent? Whatever you want to call them, they s u c k. WHM is like a healer case study in how you design a job that hates FFXIV's combat system, much like BLM is for caster DPS. It's a job that you'd think maybe works if you squint at it, and then you introduce it to....*points at FFXIV*.
What's this, slightly more potent free heals on twice the cooldown (Tetra vs Essential Dignity)? How terrible! FFXIV's combat system rewards shorter cooldowns much more than it does long ones. Slow-cast spells that have slightly more potency? Hell with that, modern fight design wants you to zip around the arena every other GCD like an AST with Lightspeed can. Somewhat stronger GCD heals? FFXIV wants you dealing damage, and the other healers are set up much more comfortably to heal and deal damage simultaneously, while the lilies are a bandaid on top of stopping and swapping between them. Asylum's trait gives you Yet More Healing? Too bad for you; FFXIV MUCH more frequently likes it when you handle damage with a snappy cooldown while the boss sits around casting/not hitting anyone and don't spend precious time dumping a bunch of useless overhealing. WHM is designed to shine in a completely different game from this one. FFXIV's combat loves short cooldowns (the first thing any WHM cooldown sacrifices for something stupid that the game *doesn't* reward like slightly more potency), mobility (lol Expedient, Lightspeed, Icarus next to....overhealing lilies), buff-stacking (lol my only buff requires me to stand still).
The reason the caster DPS have so many instant casts is because they would be unplayable without them. I've poked around the subject in the DPS subforum a few times. FFXIV increasingly isn't designed for the RPG job fantasy of planting your butt in one spot, maybe scooting on occasion, and getting your fighters to handle wrestling in a web-grease-spike pit on fire. The whole game wants you to play like an action combat sword-swinger: dodge rolling and darting around and getting your whole arena length 100 meter dash cardio going. One of the reasons I'd be down for a full (GOOD, monkey's paw. GOOD rework) rework of WHM is because its whole design philosophy is just so anti-FFXIV. Playing WHM anywhere outside dungeon Holy spam feels like rolling an acrobatics character in a game that's set in a flat plain with no trees.