Well then this begs the question: what is the reason? Please provide a quote from Mr. Yoshida or SE too.
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If you are that interested you can find it yourself, they have made comments about it and I'm under no obligation to find them for you. You think they just decided to never go the multiple jobs from one base class again on a whim?
It really makes no sense anyway with the way the game is setup, why have jobs sharing actions (that then need to be balanced for each job that uses them) when we can just switch between every job anyway. It adds absolutely nothing to the game. It would make sense if we were locked into our class choice, when you can freely switch between jobs there is no benefit to have multiple jobs share a base class and abilities.
You know, Yoshi once wanted to make red mage come from leveling and combining white mage and black mage?
Just throwing it out there, something isn't automatically a good (or bad) idea just because the devs say so.