One possible reason they're keeping DRG and NIN separate on the left side for now is that it makes gear slightly easier to design. It's hard to make generic shared gear that looks appropriate for SAM/MNK and DRG at the same time, since the former are more light armoured strikers while the dragoon aesthetic is more heavy armoured. Same for ninja vs ranged gear.

You'd think that they would jump at the chance to condense the gear categories a little more since it would give them less work to do. That they haven't means that they probably do care about aesthetic differentiation a little bit. Makes it more likely that they'll pick future jobs based on who can share gear with the scouting and maiming categories too, just to even things out a bit.