This has been discussed many different ways, and it boils down to SE released a class/job system that was both poorly planned and half finished.

If they wanted to leave it as restrictive as it currently is then every single class should have had 2 jobs filling 2 different roles on release.

If they did not feel this was possible then ACN should have been Sch or Smn only not both.

Pretty much with the way class vs job abilities are currently implemented there is no way to even give mrd or gld a 2nd job without totally redesigning all their abilities from the start.

I see no room for expansion on current classes nor do I see any plans to allow further player customization.

Rather sad.