Well while you may not have the MP now, you may have it later on.

Classes can use a lot of cross class spells, but when you change to your job, you'll no longer be able to use them. Sometimes because not viable. Like when you become a dragoon after lancer, you'll lose all healing spells, because its your job to deal damage, not heal people. I know its nice to keep that cure or raise just in case, but its not your job. There's not really any hybrid jobs, where you can do two roles, because everyone has their job, and you kinda have to rely on everyone else to be doing their jobs.