You have to always keep changing your jobs as they each have strengths and weaknesses. An example would be switching between white mage, black mage, and time mage to make the most use of the magic to augment your battle plans.

Time mage is especially one of the best jobs in the game, hastega is an increadably usefull spell, and quick is pretty much broken it's so powerful. (you get 2 turns instead of one)

Abilities from jobs that do not have the ! in front of them (meaning they are passive) also get stacked on the freelancer and mime classes, so the more jobs mastered the stronger those classes are.

Also Spellblade-Dual Wield-Rapid Fire.