While I can't find a quote on it, they have said that there is an inherent issue in adding jobs to existing classes. If we take DRK as an example, were it to come from MRD or PLD all the moves that currently generate enmity would have to be altered when equipping the DRK crystal. In addition, there is the limitation of weapons. If you want DRK to use a scythe, in the current system the only way to make that happen is to implement a new base class that uses scythes. This would leave DRK locked into using an axe or a sword. This pretty much means that splitting jobs from a base class won't happen again.

As to the Classes vs Jobs, it could be viable to keep them around assuming that the base stats given by the job crystal were simply redistributed to the base class. Then choosing to forgo the crystal would not hamper DPS/Health/Mana in such an overwhelming way. If there is no plan on doing so, then the only reason for Classes to exist is novelty. That being said, my FC has had a lot of fun using classes with cross class skills to do things that are well out of the ordinary. For example, using an Archer with Provoke and a few defensive cooldowns to tank Titan HM.

I feel that in the long run, the devs would be doing themselves a favor to scrap Classes as soon as possible in order to mainline development of new Jobs, but we will just have to wait and see.