shmerl: Sure no need to. And you can install Linux on that machine all the same. However I suspect that the vast majority of users never built anything and only buy prebuilt hardware. Same as the vast majority would never even install an operating system on their own, be it Linux or Windows.
So Valve have a chance to increase the number of PCs (Steam Machines) sold with Linux preinstalled. That's a major step to reduce Windows dominance.
Johnathanamz: I just don't see Linux growing at all on Steam Linux grows 0.xx% each year, outside of Steam I do not know how much % Linux grows each year.
People will still continue to purchase a pre-built PC with Windows installed, a PlayStation 4 (PS4) or a Xbox One.
I just don't see VALVe being very successful with having Steam Machines selling very well at all.
2016 Time will tell. If Linux after GDC 2015 in April is still at 1.xx% on Steam then in 2016 if Linux is still at 1.xx% then that means Linux will never take off, unless Microsoft really messes up Windows 10 a lot. Which so far 3 million people who have tested Windows 10 or was it 2 million people? I can't remember, but a lot of them are very happy with Windows 10.
There is even talk from Alienware, Dell, HP (Hewlett Packard), Intel that Microsoft will end up selling 600 million Windows license copies.
So time is the only thing that will tell Windows and Linuxes future. Time is what will determine the success of Linux.
If not many people start using Linux it's over for Alienware to sell the Steam Machines, then later it's over VALVe selling Steam Machines.
So 2016, maybe after GDC 2015 we will see the %'s.
This is the problem with only looking at percentages, Steam itself in total has grown from about 70 million accounts from the time the Linux client hit to over 100 million accounts now. The fact that Linux has roughly the same % as before means its growing as well.