The biggest benefit as I see it, is that the users won't have the hassle of setting something up: just install the OS and login. No drivers ppa, no recovery console, no xorg.conf patches. If this is true, Valve will do something very big for the gaming community.
I don't believe that they'll even focus on users installing the OS, just like 99% of Windows users don't install their own OS. Valve will provide the reference SteamBox and encourage other manufacturers to release their own SteamOS-preinstalled boxes, giving users hardware options.