I'd argue that this is where Apple is ultimately heading with the Apple Watch. Eventually, most will carry a watch-sized device which will interface with screens of different sizes, as needed, or not. Imagine going into a coffee house, and picking up a fully charged screen, that links to your watch as a display only, without having to walk around with one, or in your car, an uber, different offices, hotels, friends and family homes, etc. Even "May I borrow your screen", would be a much easier thing to do than borrowing someone's phone -- no privacy issues, with theft concerns minimized.