I believe the intended design was for class to essentially be a more generalized setup for solo-play whereas the jobs are there for party play or where you are meant to fill a particular role within a party or story segment. The tradeoff between job and class is power verses utility.

Trying to base anything beyond that just on what is presented is probably speaking too soon since the vast majority of what we currently have with the job system is little more than a carryover from Version 1.0. Over time things will probably fill out a bit better.