Choosing your "Main Class" Is it worth?

[Vanroe]

Leveling another class is not only a good idea, but mandatory to unlock your main class's Advanced Job, arcanist at level 30 for instance, you need 15 levels of Thamaturge to unlock the summoner (DPS) and/or 15 levels of Conjurer to unlock Scholar (healer). And yes, you want your advanced job, you get a stat bonus, as well as new Job only abilities.

The only downside of a job is that you are limited in how many cross class abilities you can have and what classes you can draw from. But to be honest most cross class abilities arent worth it without the main class trait to back them up. Exceptions being Swiftcast from Thamaturge, that is the best cross class ability and you want that.

But as others have said, Each new class starts at level 1, and will receive +50% exp from kills/Leves/FATES, mainly to make up for being unable to do the same quest twice on a character.