The main point of Steam machines, from Valve's point of view, is simply to free themselves from Microsoft. With the Windows Store, and now with increased XBone/Windows integration, they are showing signs of becoming a walled-garden, much like Apple's and Google's app stores. They could kill Steam off, and Valve doesn't like that possibility.
Steam machines are really just about turning Linux into solid gaming platform - so that people can have great gaming PCs without needing Windows. Within that, there will be as much variation from entry level to insane high end as there is with typical Windows PCs.