MCH is a mix between gunner and Machinist.
Yeah, the job should entirely go MCH and Gunner might be a job of its own.

But as Paladin is a mix between Paladin and Holy Knight, War a mix between Warrior and Berserker, DRK a mix between Rune Knight...
BRD a mix between ranger and bard, Ninja a mix between Thief and Ninja...

Job mix exist to satisfy two needs in one go and that's what happened with MCH.