That's an intriguing concept, but it seems like it would be a headache to implement, from a software perspective. Unless I'm mistaken, if you wanted this low-power chip to perform windows updates, you would need MS to provide an ARM compiled program to perform x86_64 updates. Or you'd need a third party to do it somehow, which sounds messy. The same would apply for anything else you wanted to do that's more complicated than putting files in fixed locations. I suppose it could be relatively easy to just do the downloads, and leave installations/updates for when the main chip kicks in.
And the fileserver concept is pretty neat. Rather than having a separate low power machine that you send files to, your main PC could act as a fileserver using separate software while fully powered or in low-power mode. I'd buy that, if only for the novelty.