I want to play the game long term

[Reinha]

There may be a good reason to purchase a single job boost: you gain access to all the level 50+ starting jobs.

This is a great point. Many of the older jobs are clunky and boring compared to other jobs of the same role. With a single job boost you can unlock 2 tanks, 2 melees, 2 ranged, 1 caster and 3 healers (10 out of 19 jobs!) without having to do MSQ on a job you don't actually want to play. By boosting a SMN/SCH you also get two jobs leveled up for the price of one.

A newbie would be able to try out a ton of jobs and pick one to do MSQ with, and they would still be learning how to play because the story and all related instances are where the gameplay and mechanics are taught.