It's just my playstyle, but I couldn't continue past 30 until I unlocked the job for the class I was leveling. I don't know why people don't want to out in the little effort to improve their class/job, but to each their own.

I've ran into my share of jobless people as far up as Stone Vigil. There is an obvious difference when that gla left and we got a paladin. It was extraordinary. If I remember properly, Yoshi described a job as something to use when in a party setting, and the class for solo play. This was back in 1.0 though. Same concept applies now, but jobs can still be more effective than the base class regardless of the "extra" ability slots.