It was originally described as classes are for solo play, jobs for party play right? But the way the game is currently, you don't really need to switch back to a class when soloing.

I think an idea could be to add some challenging solo content where the flexibility of more cross class skills would be more beneficial than the higher damage stat bonuses jobs give. Of course, that brings the potential negative effect that people will have to lvl several classes they might not like. Maybe in the future as new classes and abilities are added, it will be worth it to play on your class.

(I will say, while I'd rather be rogue than ninja, I REALLY miss my carby on arcanist ): )