As someone who has quite a distaste against WHM/RDM, my opinions here may be way too biased towards /SCH, but they're that way for a reason. Saying that from the get go, and I'm only going to cover the subs I've commonly seen a back-line White Mage use.

Black Mage

Before I even start with /RDM I am going to comment on /BLM. Support jobs are meant to enhance your main job. Back before Scholar was created, this was either /Black Mage or /Summoner. Warp is not a valid excuse to /BLM (neither is Warp II), it does nothing to help your main job. It's a convenience spell, similar to teleports. At this time in the game, the only valid reasons to /BLM are Stun or Sleepga, aka: spells that are helpful in situations that justify it. That's it. MAYBE Tractor outside of Abyssea, meh. Moving on.

As for convenience spells, I say [Instant Warp], [Warp Cudgel] [Do you have it?]... It's really that simple. Then again, I've developed the habit of having those on me, so I have my own point of view on it.

If you intend to use Stun, or Sleepga when the situation arises, it's fine to use the support job.

Red Mage

Oh boy, this is the touchy one with me. Maybe it's because of the fact BOTH VW and Abyssea require Wings of the Goddess, meaning ANYONE that does either event is capable of a) unlocking Scholar and b) leveling it and c) subbing it like they should be. I see nearly no reason to ever use this support job, it offers nothing unique to White Mage that actually helps it out. It's a MP battery support job, and a darn good one... Scholar still beats it. Its only niche is 5% extra Fast Cast, something I definitely don't consider vs. /SCH's overall usefulness. One may say you get MP immediately using Refresh+Convert, that's true. However, I like being able to save my overall MP and use it more efficiently before I even run into a problem like that, something /RDM isn't even good for (and what /SCH is capable of).

Okay, maybe there's one perk... easier access to Dispel. /SCH requires Dark Arts and Addendum: Black used before you can even cast it.

Short summary: If you /RDM when you can /SCH, I don't know what to say. My "Huh!?" moment is when I see WHM subbing SCH in VW JUST for the helix spells (Yes, I've seen a few. Before they lowered the levels on them, these WHM were ALWAYS /RDM), because it shows they don't use the support job frequently and don't see what's so good about it.

Now for the nice part of my post...

Scholar

Easily, by far, the best support job. /SCH does what a support job is supposed to do for mages, directly enhancing their ability via usage of Light/Dark Arts. Each arts lowers MP cost and casting time by 10% for their respective magic while bumping skills up to at least B+ ranking, effectively buffing not only your casting ability, but your efficiency with magic skills if they aren't up to date (some are buffed regardless: Enfeebling and Enhancing magic).

Penury/Parsimony lower the MP cost of the spell by 50% of its natural cost, not 50% more (to -60%). Conserve MP, the other perk of /BLM back in the day, also applies to /SCH, and it can stack on top of Penury/Parsimony. Celerity/Alacrity do the same to casting time, reducing casting time and recast time both by 50%. Accession makes certain spells AoE'able, and Manifestation can be used with Aspir for interesting MP returns when the situation would arise.

Sublimation is nice too, effectively the same as Refresh @ Lv90+. The only difference is it's stored over time vs. getting the MP right away, but if your MP is full Refresh's essentially wasted while Sublimation is not.

Basically, once I used /SCH, I was glued to it. The level cap increases haven't changed a thing - they've only made /RDM more of a viable option then it was at level 75 (essentially worthless).

You also shouldn't /SCH just because you gain helix spells for VW.

Summary of my entire post: /SCH is the best, it's that simple, it offers the most to the job and it should never be overlooked.