I think you're somewhat ignoring the differences that do exist between the jobs and only pointing out the similarities.

That said, the issue with making a job too distinct is there will inevitably be classes that are much better suited to certain fights than others which could lead to jobs being ostracized (very common in FFXI). The way the jobs are tuned now, for better or worse, is much more balanced in that you can essentially do any fight with any combination of jobs. Their are some that are stronger in certain fights than others but never enough, in most cases, to lead to some being excluded.