What consumer is Valve aiming for exactly and are those consumers numerous?
The millions already on Steam who (probably) game on capable desktops who still wouldn't mind gaming on a TV in the living room. Yeah there is nothing stopping anyone from making a HTPC and using that, but there's nothing stopping Valve from trying it themselves, either. They are flush. It all depends on price and specs.
And maybe a device like this would allow simultaneous access to a single Steam account across multiple devices? Prez could play Snuggle Truck on his PC while his kids play his other 1700 games in the living room.