A high main class will aid in unlocking new content (Content is usually unlocked through MSQ progress, which has a level requirement) and some of this content, such as beast tribe dailies, wondrous tails, and custom deliveries, are things that give a lot of rewards but have limits on how often they can be done (either daily or weekly.) For this reason, people usually encourage getting a single class up to unlock them so you can do this alongside leveling your side classes.

That said, if you feel like you wish to level other jobs, by all means do so. You don't lose out on anything, at worst you're just putting stuff off to later. Also, if anyone mentions the Armory Bonus, that's often ill-understood: It will give an xp bonus to any class lower than your highest job. You don't NEED a job at level cap to unlock it.