And you have to assume a mobile device will stay at home so it can be used like these (conceptually stationary) things. Once can use a mobile device as their TV too but I bet most people have a TV set anyway. One can argue it's a whole computer too but I bet many have a laptop and/or desktop too.
iDevices are a good shot at do-everything devices... but they are intended to be MOBILE. For the single, living-alone, that's no issue at all. For everyone else, the desire for generally stationary can trump the utility of using a mobile device like it's a stationary one.
It's like the very oft-thrown argument of using an iPhone as a remote control. Great if you are single, living alone. However as soon as the "good remote" leaves the home with you, the others back home no longer have a "good remote."