Why do that when they can just release the Job system which comes with it's own skill set and Job Specific armor and accompanying quests? I'm guessing you asked this because you don't want to grind, which is cool, but the job system is more than just to give an identity to things.
The main purpose is for party based content and to give some sense of definition -- just adding it to classes won't lead to definition, because for example a conjurer, while is still a healer can also still nuke within it's control of elements -- a White Mage however will have a higher more specified focus on healing which means being a WHM you'll be more in line with healing than a conjurer is which is in line with nuking and healing, especially for solo but for party play you'll wanna be your WHM rather than CNJ.
