don't you know you can level all class in this game and some classes share gear set with other classe from same role?
If my BLM equip a book he keep all his gear and have a totally new skill set (the SMN one)
if my whm equip a book he become a sch but keep gear set
my pld can't equip GA but it's only becaus i havent leveled war yet.
ROG/NIN might share gear with some existing class (PUG/MNK?).

All you ask for is the current existing armory system without having to bother leveling other classes, and more multi class EG Gear