The notion that a single-machine household is the norm sounds a bit outdated to me, and the general population are anything but effective users when operating manually.
The young adult homes I visit contain at least 2 PCs each, often more (1 desktop, several portables), multiple multimedia devices, and at least one digital camera. Typically the desktop is used as a data dump, chat station and (highly versatile) media player. The backups are on a flaky USB drive. I know a lot of tech-savvy folk, so the desktops typically stay on for hours - sure, messenger apps, internet surfing, games, videos and music are not NECESSARY, but people certainly do enjoy them.
Compared to browsing and chatting on tablets or netbooks (or smartphones) and using a NAS for data and media, that is simply inefficient - especially with the long uptimes and lax default power-saving schemes.
Data hoarding is real, but it has its upside: shared culture is preserved culture. I would also never give up local copies for clouds or streaming - the only real ownership is full ownership, and data can only be guaranteed to exist by making multiple copies of it. Even some contemporary works - never mind your personal files - can only be found in people's private (but shared) collections.
I think it's great how we have the same goal
(green, lean homes) in mind, but have different approaches
: minimalistic/solitary and diversified/network.
My view is that a NAS could serve as the mainframe in a home where now a desktop behemoth is used instead - not to mention various players and flaky auxiliary storage devices. Data storage, backup, sharing, and streaming, in case of media files, are more efficiently handled by a NAS compared to a desktop (and a selection of peripherals). By using a NAS, desktops could be reduced to something like the Intel NUC, or, in case of gaming and work machines, a mini-ITX rig. This would save a lot of resources (materials, energy, even precious square feet in a home). In my own experience, many homes would benefit from this arrangement even today, as terabyte-class capacities and multi-device access are required (and power is wasted), not to mention terabyte drives having all but become the standard in HDDs.
The minimalistic approach has strong merit for single-user, low-intensity scenarios, no doubt about that. Why buy a farm if you just need tomatoes. However, for the digital home of tomorrow, I see the diversified device structure as the optimal solution, at least until we get PCs with quantum storage and infinitely adjusting performance.