The only real benifit to mastering jobs is the transfer of abilities to your Bare character, which is great late in the game. The best way to beat the Final Boss is with Bare. However, early in the game, you want to be earning AP, so equipping Bare is pointless. Therefore mastering jobs right away is unimportant - especially since for the mages, you don't have the MP to spare for the higher level spells yet or even the locations to buy them.