From what I can tell, I'd say it is more of an interim compromise. Ideally, they would have the entire existing steam library running on SteamOS. But obviously they can't do that, since many companies simply do not wish to go through the hassle of porting their old code to a new OS. So, streaming from a Windows box allows them to support the entire existing steam library as more games are ported and created for SteamOS. Also, I am guessing they will do some sort of input passthrough, so your steambox will process the input, stream it to the Windows box, and then stream the video and audio back to the steambox. This should allow you to put the Windows box anywhere you please, out of sight.
All in all, it sounds like a neat idea, hopefully they can keep input and output delays from the streaming box low though, otherwise some of the more hardcore gamers might get annoyed with it. But again, this seems like more of a middle ground compromise to please people with existing libraries, they even mention announcements of upcoming AAA games for SteamOS in the next few weeks in the link.