Ok, lets take a look at some of the top speed runs done in fflogs by SCH/WHM combos and compare their actual total oGCD healing done in a savage raid situation.
https://www.fflogs.com/reports/F7qar...pe=damage-done
WHM - 1670000
SCH - 1488400
https://www.fflogs.com/reports/jv3Wz...0&type=healing
WHM - 1202900
SCH - 1457000
https://www.fflogs.com/reports/xyHXJ...pe=damage-done
WHM - 1533000
SCH - 1832600
https://www.fflogs.com/reports/dcvZg...aling&source=5
WHM - 1525800
SCH - 1379800
WHM average - 1482925
SCH average - 1539450
WHM wins on 2 of those runs, while SCH wins out on 2 runs. The average done by SCH is slightly higher over those 4 runs by ~3.8%. But this doesn't take into account the HoT utility provided from Medica II because it was too hard to separate the hot portion from the actual base potency. Both healers end up having similar oGCD healing numbers in practice, even in the most difficult savage encounter.
https://www.fflogs.com/reports/NPyVd...8&type=healing
Fastest WHM/SCH o3s run -
WHM - 1144900
SCH - 989400
WHM wins out here as well with the fastest o3s SCH/WHM run I could find.
It's nice to talk about how awesome SCH oGCD's are, and I won't deny that they are awesome, but WHM bring just as much potential to the table in raid encounters when they are forced to use their entire kits.
The flexibility of SCH oGCD + chain strategem is the reason SCH are preferred as the 2nd healer, because SCH in their current state depend on their co-healer to carry the brunt of all GCD healing.
Anyway, like I said if you buff WHM's damage by 10% or so while giving SCH the ability to carry GCD healing, you can start making a case for bringing more WHM to the top raids. Right now the utility of Chain Strategem and AF stacks is causing groups to use only AST/SCH combos with WHM being left out most of the time.
I actually had to go to the 2nd page to find WHM/SCH combos for o4s because they're so ridiculously rare.