Destiny's Device - A New/Updated Rune Fencer Guide

/WHM (but /SCH is a perfectly valid alternative) was my subjob of choice on RUN for a large variety of content but if we're talking about recent ones I could list 8 Sortie bosses with Mage setup.

In Sortie you don't need to supertank, as such you don't need the boons from /BLU (mostly cocoon and AoE tagging).
To be fair, /WHM can be perfectly fine for tagging as well, you have Diaga and Banishga.
Can heal yourself and others just fine with a decent Cpot capped DT midcast set. Not that there's much healing involved in those sortie runs tbf...
It's also nice for sneak/invi and for -na

Nice I'll check it out. I think there's a few other misc situations I could make use of /whm over the more common choices. Are you curing in Sroda belt too? I feel like the mp management shouldnt be too hard and it frees up a lot of other dt slots.

Aye.
Hard to cap DT and Cpot without Sroda Belt. Is that even possible at all?
As I mentioned before you have your own Refresh, idle Refresh and if things really go wrong you can Umbrax3 and Vivacious Pulse for MP.
But like I said there's not that much healing involved at all, as such the additional MP cost from Sroda is not really an issue.

I think /WHM and /SCH kind of serve two different purposes. /WHM is for group healing support (Curagas) while /SCH is more for utility (e.g. Sneakga and AoE ailment removal). If you don't need to heal the whole party, then it's hard for me to imagine a scenario where you'd want /WHM over /SCH. /SCH is basically just better unless you have a specific need for /WHM such as, again, Curagas.

In terms of Sortie, RUN/WHM is a viable alternative to PLD in Aminon melee setups (where the tank doubles as the party's healer).

As for RUN/SCH, like Sechs pointed out it allows you a better ability to heal yourself compared to /BLU which can lessen the burden on your other mages. Yes, people who buy/bot MLs probably have access to Magic Fruit, but /SCH also gives you ailment removal without relying on meds or the 60 second Vivacious Pulse ability, Reraise, Sneak/Invis, and a few other tools. Additionally, you can Sneakga your party after a boss as people gather for Bolters and maybe save them some meds, as well as helping remove ailments at times (like Accession Erase after C/G to remove peoples' Bind). I really like popping Accession on mini-bosses (Botulus/Naraka/etc) so that I can Sneakga the party the moment it dies. None of this is game-changing, but it's convenient and makes you more self-reliant.