Before SCH came out the only subjobs my WHM had are BLM and RDM, lacking SMN.  And even at this point, SMN subjob will remain at 15 for now until I see a real reason to level it.

I'm not *entirely* sure on SCH's stat boosts as a subjob, but that barely even matters.  SCH right now is by far THE most useful subjob for WHM.

/BLM offers, uniquely, right now:
 - Elemental Seal
 - Warp, Tractor, and Escape
 - Sleepga?

/RDM offers, uniquely, right now:
 - Enspells.
 - Phalanx.
 - Slightly better fast cast when it comes to cast times.

/RDM and /BLM offer, generally, the same black magic enfeebles so that's not included.

/SMN offers, uniquely, right now:
 - Auto-refresh?
 - lolAerial Armor.
 - Shining Ruby isn't too bad.

/SCH offers...
 - Light Arts.  This is one of the best things WHM can benefit from that can far outweigh the benefits of the other 3 subjobs.  This boosts your Enhancing and Enfeebling magic skills to 256 (max).  This is really useful for a few reasons.  First, you gain slightly greater barspell power.  Secondly, Stoneskin takes even less MND to cap at 350.  Thirdly, enfeebles land a lot more reliably due to the boost and respective gear.  Divine and Healing magic are unaffected.
 - Under Light Arts, white magic spell cost is reducted by 10%.  White magic spell casting times are also reduced by 10%.  The recast times, as well, are reduced by 10%.  In contrast, black magic spell cost, casting time, and recast times are all increased by 20%.  An effective way for SE to say "Do not cast black magic with light arts up."
 - Penury (SCH10) and Celerity (SCH25).  These abilities use Stratagem Charges, and a SCH37 has two of these.  Penury lowers cost of your next white magic spell by 50% (does NOT stack with the -10% natural cost from Light Arts), while Celerity lowers casting and recast time of the next spell by 50%.  The charges take 2 minutes to recharge at SCH37.
 - Sublimation (SCH35).  This makes /SMN 'lol' for the auto-refresh.  What happens here is that HP (drains at 2HP/tick, absorbable by Stoneskin; able to wake you up from sleep if Stoneskin is not up) is 'stored' for a time and you can use it again to gain the amount of HP stored as MP.  The amount of MP gained from using this easily out-weighs Auto-Refresh from /SMN.
 - Dark Arts can be used, with addition of dark magic gear, to have decent returns from Aspir, effectively allowing even MORE MP recovered.

WHM/SCH takes more effort to play effectively, but the effort is far worth it as it has single-handedly dominated all other subjobs for WHM.

/SCH has officially made me think of the following subs in the following manner.

/BLM - Warp.  Elemental Seal won't be needed in a lot of situations; neither would Sleepga.  Tractor/Escape could be situationally useful, so it's still not a bad sub to have.  SCH also has Conserve MP now, so it's a moot point.
/RDM - Still shines with the fastest casts, with the exception of when Celerity is in use.
/SMN - Max MP only(?).  Other than Shining Ruby's situational use, I can't think of a situation where I'd sub SMN on WHM in most events.  Aerial Armor is so 2004, it's 2008 is where utsusemi is king.  Sublimation completely destroys the benefit of Auto-Refresh over time, as Auto-Refresh is only 20 MP every minute, and abusing Sublimation can easily gain more over time.  Not to mention you can use Dark Arts and get decent Aspir returns.