I agree with most people that you're a little bit way too pessimistic about WHM, Tonberry, but in a way I can get behind giving WHM at least an answer to Collective, Soil or Fey Covenant. However, this...
This right here. This sentiment, I think, is the most important! There is no real "oomph" to WHM, and it's kinda sad. It's, however, not the biggest issue I have with the job.
I've played all three healers in raid this tier. Started as SCH, then switched to AST, then did WHM for a good chunk of v4 prog. Finally switched to SCH as a main (even if forum says it's PLD :P), and honestly, the one thing really, really bothers me about WHM is the lack of action efficiency.
What really turns me off of WHM is that its oGCD heals are rather underwhelming compared to the other two. In a game where GCDs are the most precious resource, this really hurts the job.
Assize is the only true oGCD burst heal they have, and though its healing potency and CD is warranted due to its "secondary" effects (the DPS and MP restore), it still pales in comparison to Indom and specially Earthly Star.
Aslyum is also... odd. If Collective Unconscious went back to its initial incarnation of having the effect change between sects, I would say Asylum is fine as is, but right now... not really. In my head, CU and Asylum serve more or less the same end as Whispering Dawn: a "free" HoT CD that costs no MP and GCD. However, Asylum is by far the weakest of the three. It has the weakest potency per tick (only if we count WD with Rouse, which should always be the case), and it has the disadvantage of needing everyone to be inside the zone for the full duration to take effect. CU, meanwhile, can have everyone stack for a max time of 3 seconds, get the regen, and then go on their merry way, while the strongest oGCD regen in the game ticks on them for up to 25 (!!!) seconds, while also mitigating damage at the start. Whispering Dawn can be triggered even in the middle of casting and has the same range as Medica, which can also be manipulated by fairy positioning, for the better or for worse (this is really handy in 'spread out' situations like Fire IIIs).
Plenary Indulgence is... kind of an oGCD? The fact that it absolutely needs a GCD heal to be cast to even become available kinda kills the purpose of an oGCD, but I won't deny it's a nice tool to have.
We can argue that it's pointless to give WHM more HPS than it already has. Yes, they have more than enough right now. But honestly? Why the hell not? They're the so-called pure healer, yet have the weakest GCD AoE heal in the game ever since the buff to Succor's shields. Asylum is half as useful compared to its alternatives, Assize is the weakest oGCD heal too, and while PI gives them an edge where burst healing is absolutely necessary, can it really compare to the efficiency of Indomitability and Earthly Star paired together? The answer is no. No, it cannot.
Honestly, boosting WHM's burst healing potencies would only help them to use less GCDs healing and more GCDs DPSing, which is, atm, their only way to rival rDPS contribution by SCH and AST. And also it would serve a little bit to really, actually solidify it as the "pure healer," even if it's not really necessary. If it makes WHM mains feel more powerful, then to hell with what's "necessary."