This is how I sed MP being use for each Class:

Bard: uses MP to buff the Efficiency and Durations of their Songs, Skills like Troubadour for example should ne used for this
Machinist: uses MP to convert it into Ether which fuels the Energy Bar to call Mechs AND Turrets, both should be useable together, bring back ground targeting, aoe turret und its explosion skill, Remove just heat and rework the skills that required it, its currently just 1 - 1 gameplay mechanic for a single skill is pointless
Dancer: same as bard, buffs dance efficieny n durations with mp costing skills
Dark Knight: Specific Attack n Self Buff Skills cost MP
Paladin: Specific Attack n Self Buff Skills cost MP as like more powerful Tank Skills to buff Defense Powers of your Group and to protect from lethal damage, that would otherwise kill others
Warrior: Uses MP for self buffs to boost up stats, max health ect to become the true Berserkers that they should BE, also as Job Name, warriors are we all
Gunbreakef: Converts MP into special Magical Bullets, that if used, deal more damage, have more range und can cause sideeffects
Monk: Uses MP for Elemental Stances
Samurai: Uses MP, for gap closers, and as additional activatable buff up cost of Kenki Skills to charge Kenki quicker up in have more potence as lobg you hav mp
Ninja:Uses MP for Ninki Skills and to convert it to required Chakra to perform Ninjutsu with Mudras, due to the usage getting jore limited with Chakra, Ninjutsu become more powerful to compensate, us for gap closers as well, ninjas should be the most mobile job of all
Dragoon: Uses MP for Jumps, making them more mobile ad well, nearly same as like Ninke, less often, but longer distances, Activating Dragon Powers should cost M
Casters of any sort: Uses MP for casting Spells

Like said, basically should every class use mp, there is no good reason why this shouldn't be the case, its only a matter of skill design n balance