From what I understand, by completing a specific Job quest, you will then have access to the job. So a R50 Archer, after completing the R30 Bard Quest, will be able to be a R50 Bard. But there is still the necessary quests to acquire the specific Bard skills from 30-50.

Now, the real answer lies in how difficult these quests are. If they are in fact difficult, then the growth from Class to Job will be meaningful. Also a great deal of story could be added to give them depth as well.

So here's hoping these trials/quests etc are meaningful/challenging/story/cutscene-packed for maximum awesomeness.