The benefit is receiving security updates. People may not choose to update their phones with security in mind, which is all the more reason to do it. Security updates is a place where consumers can be shortchanged simply because they are invisible, the consumer may not be aware that the security of their phone has been breached, and it is the sort of thing that consumers rarely think of until something bad has happened.

As for cost, I don't see why it would have to go up all that much. Apps are already upgradable on phones and much of the OS is hardware independent. So the only real pressure point is with the kernel and other hardware dependent code.