the problem i see personnally is that you can't put all the ressource on the same device.
if you want your phone to be the brain of everything it's doable BUT the laptop extender should come with power on it's own that add to the phone.
i mean when you are not in a "PC configuration" you don't really need your phone to be the most powerfull device of the universe.
a snapdragon 800 is enough in a phone for pratically any use, if you keep in mind it's a phone.
when plugged in a laptop extender, the extender is where the strong processor should be located. (not sure i am making myself clear).
as long as you want the phone to be the brain AND the powercore of a multi device you'll end up with a not powerfull enough device to be a real PC or laptop remplacement. (like the padfone of asus)
for me the better concept for a multi device will ever be this
one