To be honest, I think it's more because they refuse to give two classes the same spell and that (this part is sensible) they don't give jobs traits to support cross class abilities. It's why there's Cure/Physick Raise/Resurrection. They couldn't justify a clone of protect just to justify sch getting proshell. SCH is not very inferior to whm, it isn't really at all, it's just different, but it does have some flaws (like Adloq not being stackable, not the best pet AI) and some boons (never runs out of mp) and the value of not having proshell is rarely noticed because you usually have a WHM as well.
That said, I like both. For me SCH is more fun, because dps'ing seems more...in tune with the class. Stone II is boring and Holy is too expensive on mp.