The bigger problem is that jobs with branching specs is an entirely different system than the one that they are building off of. Or more specifically, it's different than the system they are building away from. Jobs were originally the specializations of the classes, but because of how stat distribution works this didn't quite work out as intended (they tried with summoner / scholar and we saw how that turned out.) To make different specs, they'd need to allow multiple sets of stat point distributions when using the same base class.

Don't get the wrong idea: I'd love to be able to have a choice in how I can play monk or Dragoon rather than be stuck with just one way to play the job. However, that's basically asking for multiple flavors of the same class, which would add considerable workload to the dev team who are already busy bringing samurai and blue mage to the game.