Man, that would be great. Imagine going out for a walk with just your smartwatch. Then you go back home and you insert the watch into a phone case and it gives it more RAM and CPU power and it becomes your phone and you go out again. Then when you come back you put it inside your big box and now you have a computer. All with the same accounts, OS, apps.
I mean, we already basically have that but even more convenient -- you don't need to have a watch-shaped hole in your phone, or a phone-shaped slot in your computer.
I already use all the same accounts and apps and data across my watch, phone, and computer. I don't particularly want to take my watch off to use my phone, or put my phone away to use my computer though.