I always found the excuse a bit lackluster. You are not leveling 18 different characters, you are one character leveling several jobs. If you are a level 50 black mage, and thus can wear level 50 black mage clothes, you shouldn't be unable to glam them just because you are currently holding a sword in your hand. You are still a level 50 black mage imo. (i.e. let us glam anything we can equip on any of our leveled jobs)