I don't get how using XI's subjob system would be an improvement over the class system. If you feel you need every cross class ability under the sun then just don't wear the Job stone. Simple as that. The Jobs are supposed to be specializations on the class. It's why you lose half your cross class slots and are restricted to what classes you can cross class from. XI's sub job system would not be any different. It'd actually be worse as you'd then only be able to draw from one class/Extra Job for your cross class abilities instead of two.