It's definitely neat, but I feel like this has been done before without success -- examples like the Motorola Atrix, Windows Continuum, and even the cancelled Ubuntu Phone come to mind. (And, going further back in time, let's not forget the Palm Foleo!) It seems like a neat idea on the surface though, so I wonder why as a concept the phone-connected-to-desktop-peripherals thing hasn't gained more traction. My suspici…
I think there's a slightly different model that makes even more sense ...
What we see with DeX (and Altrix, etc.) is the phone as the CPU that gets plugged into larger devices.
But what I think makes more sense is a smaller module, as the CPU, that gets plugged into larger devices including a phone.
So instead of plugging your phone into your "PC" you would move the core module from your phone, to your PC.
That way the core module can be much smaller and can power future phone upgrades, etc.
I think a PCMCIA[1] card (a "PC-CARD") would be a great form factor for the compute module...
[1] People Can't Memorize Computer Industry Acronyms