I for one know a lot of people who would have quit if they took away the ability to equip any subjob you wanted in XI even if it didn't take away the specific subjobs they used themselves. I would have been one of them, because some of the most fun I had in XI was when I played Job Combinations that most people ignore. Even if it was only for a specific event.Wheres your freedom of play now?
White Mage/Paladin - Tanking Bones for a Mnk/PLDx5 Party.
Ninja/Ranger - Dealing enough ranged damage that I could tank without war.
Blu/Pld - Shield Bash works even if its on a Alljobs Shield.
Samurai/Drg - Yay /drg gear.
There were tons more too that I never got invites to parties for, but myself had fun playing as.
Any limitations of that nature should be implemented by yourself or the community and not the game developer. Thats the nature of a sandbox.
The School Builds the walls, it fills it with sand and toys but The people playing in it make the rules within those walls.
The problem with your system is you are trying to make the sandbox small then we already have access too.