I think basically what you are trying to say "Why have classes if jobs are the only real efficient way to play the classes."

Short story: We are essentially stuck with it since that's all Yoshi had to work with. I would imagine it would have literally been a large under taking to remove/replace the current armory system to something more fitting ARR. I like the concept. The problem is that classes are almost useless once you unlock the job. By the time you get your job, the game becomes less solo oriented and more focused on party play which is where the job specializations are meant to be played. Even outside that, jobs are still better than playing a ass because if the job abilities.