Banking is probably not quite the right word, but overall MCH's slightly better mobility is a function of two things. More ogcds in general which makes it more likely you will have one up to cover a period where movement is required, and ammo which is a controllable resource.
I do agree that they could do a lot more with ammo system - ammo is basically the MCH shtick. That said I don't think they tried particularly hard to make the MCH extremely distinct, the job exists more to just give someone who wants to play as the battery another aesthetic and gameplay option - if you don't like the court jester aesthetic BRDs have or think they're too DoT reliant, then at least you have MCH as an option.
If the 'support ranged DPS/mana battery' is a clearly defined role they want to have, then it's inevitable that there will eventually be more than one take on the archtype. It feels no different to me than how PLD WAR DRK are basically slightly different ways for you to tank, or BLM SLM are two different takes on casters.
