Those of you who want the option to use all jobs on all classes are not looking at this realistically, the option u guys are asking for might be nice to have and all but it won`t be in SE`s best interest to create such a system. Think about it for a sec, even with only 1 job per class, there is a lot of balancing that needs to be done but if they made jobs free for all, they would need to balance all jobs for all possible combination and this would apply every time they want to add new sets of jobs and i don't think this will happen. It`s to much work for a team to commit to when they are trying so hard to fix a broken game and such a system will only create more balancing issues.
Also based on what we know now, the chance classes will be locked is high, on top of the above reasons, they mentioned that they are working on Job Specific armor. If classes are unlocked that would mean Archers and Conj can run around in Paladin and Dark Knight gear just because they have the job equipped and i really doubt that's gonna happen. The way i see it, Jobs will be locked to specific classes and based on the Dengeki interview, they are only adding 1 job for each class to start. Maybe in the future we might see new jobs but they too will probably be specific to a class.