I think most people are missing the point. It is not about replacing your main OS or PC it is about having platform independence. The same reason why Google bought/started Android applies here; protecting/getting the means to sell your services. The Steam Box is going to be an open h/w and s/w platform. If you would like to integrate Steam in your TV? please by all means! You would like to sell your own Steam Box? How may we help?
People that are going to buy a steam box are people that would otherwise buy a ps/4 and/or a XBOX one. Nobody cares that the PS/4 is running BSD nobody is going to buy a PS4 because it runs BSD. Microsoft failed to understand this as well, "we have three OS's running on the X1", yeah that will sell a game system like cupcakes. Almost nobody is going to buy a game/entertainment system based on the OS. It boils down to cost, marketing, what your friends are getting, launch games and exclusives.
If the steam box is going to have similar h/w and price point as the PS4 and X1 and a couple of cool new games like HL3, it will have more games at launch than the PS4 and X1 combined (and as an extra bonus you can play your games on a PC running Windows or Linux). You want to play HL3 on a console? buy a Steam Box.
As for bu-ray support, if it turns out to be a main point of consideration they could make a player that support it. But looking at the PS3 vs X360 it wasn't a main seller for most people.
TL;DR
it is not so much about all desktops running Linux but most of everything else