Class => Job => ???

[Zojha]

now you can balance this job without touching the base class and you can add as many jobs as you wish on that same base class.

And the question becomes: Why even bother tying it to the base class then?

At that point, you have effectively two separate jobs. The only benefit of tying them to a base class then is to reduce the leveling load for leveling all jobs because you level two for the price of one. You could just adjust XP gains and requirements instead, same effect.