I'm concerned primarily with WHM's use as a support job in the post 75 world hitting us this summer. Obviously anything that we can discuss now is largely speculation, but it's something I was discussing with some friends last night, and I was wondering if anyone else might have a different take on it.
My initial thought was that, aside from the convenience of teleports, past level 77, /WHM isn't going to be offering an awful lot to RDM. At level 78, you can get erase from /SCH while also getting the obvious benefits from Light Arts and associated strategems. Sure, WHM gives bar-ra spells, but I don't find myself having to use these with much frequency (more often than not, if it's a situation that actually calls for bar-ra spells, there's already a WHM doing it anyway).
From 38-49, the only new toys offered from /WHM are Afflatus Solace and Misery and Repose. Afflatus Misery is relatively useless for what I feel are obvious reasons. Afflatus Solace offers the interesting aspect of the stoneskin equal to 25% of the cure, but I don't foresee that alone justifying the support job over such wonderful abilities as Accession.
Repose was initially the more compelling reason to use RDM/WHM once we get it at level 96. Further consideration, however, made me doubt its utility. Certainly having access to both light and dark-based sleeps is useful in certain circumstances, but given our E rank divine magic skill, I imagine it will be relatively unreliable against enemies from the new content even if you're very well geared. I imagine that the skill growth patterns will be adjusted past level 75 (just as they make changes at 51-60 and 61-75), but even when I'm making conservative guesses, I'm imaging that E rank will be at least 100 points below our A+ ranked enfeebling magic. Beyond that, if we're really in a situation where sleeping is a concern, /BLM is probably preferable for sleepga or even /SCH for Accession Sleep and Sleep II. If light-based sleeps are truly a concern, I don't really see RDM/WHM becoming the go-to job anyway.
In situations where the level 99 E rank skill isn't as much of an issue, I still don't foresee much usefulness. At level 99, even E-rank will likely be somewhere close to current A+ rank, but given all the other tools we'll have available, I sincerely doubt that any of the existing content is going to be so meddlesome as to warrant the use of both light and dark based sleeps.
tl;dr version: I can't think of any good reasons I'm going to need RDM/WHM past level 77 and do not plan on leveling the job any further as a result. Note any pros that might make me reconsider the usefulness of the sub beyond convenience.