Valve can use Half-Life 3 / Portal 3 exclusives on SteamOS to push adoption of the OS/console which benefits them greatly.
Back when HL2 came out, I didn't want to install Steam, but ultimately I wasn't going to miss out on the sequel to the best FPS ever made. Then again, it's far easier to install Steam than it is for the average person to abandon Windows. I guess that's where the Steam Box comes in.
If you say that SteamOS means a game is Linux exclusive, you can say Metal Gear Solid for was CellOS/BSD exclusive[0].
[0]http://en.wikipedia.org/wiki/PlayStation_3_system_software