The SIM offers flexibility.
I guess it's the level of control and your experience. I support Blackberries for instance and keep a spare SIM for activating a users replacement... meet them, swap SIM, job done in minutes. No end user hassle. Similarly with swapping devices,,, copy numbers to the SIM and transfer. I do know some carriers are making this easier but haven't experienced that yet; I would say that I would miss the simple SIM card a lot. Doesn't affect my Apple support as they dont allow you to copy data to the SIM, but they are happy to inport. Wonder why??? Surely not to make swapping devices more work that it needs to be???