Do you read what you type? Srsly.
I do not ever run out of MP, I can keep DD's alive as well as a WHM, while also providing things a WHM CANNOT provide to increase their dmg/survivability, modus veritas procced like 5 NM's for me last night,helix spells are procs(I procced like 8+ helixes last night), and my LS loves embrava durning "oh sht" moments. A 3rd WHM is a complete waste of a slot, especially on some of the harder NM's where you need as much damage as possible within 30 minutes. We almost timed out once, and only had a few minutes to spare the other times a lot, replacing me with a WHM is a horrible idea. Libra also is very useful for DD's to know where they are on the hate list.
I also do tons of damage and provide procs a BLM can't provide no matter what, if I'm not healing and nuking instead I give all black mages 15% additional damage + klimaform....... Clearly, replacing me with a BLM is better than giving 4-5 BLM's +15% dmg
SCH is amazing, you're just probably bad at it (and this game)