The only thing I don't like about the "Advance Jobs" is the line up. Say RDM uses BLM and WHM as the two core classes. It HAS to select one as the base so either BLM or WHM sets the standards, so Armor set and Weapon are set to that base class as well as all cross class abilities would be restricted just to what the 2 core bases revolve around. Since Job changes with each stone equipped and not part of the cross class, you don't get any of the 5 bonus skills the original jobs had without rehauling the system to support the new Advance jobs.
If Advance Jobs were a key point, and please note, you don't even have to play the jobs needed, just unlock them and level the base Class to the appropriate level unless you quest lock the job quest and skill quests for it. Also leveling up, if left to the same standards would only level the base Class which then would raise the level of the first Job, which would raise the level of the Advance class.
Even if they were to add 2-3 Advance jobs at a time to help with balancing the roles properly, you could level all the current classes to 70 and essentially have all future Advance jobs at 70 upon release anyway.