What? Why? If Steam releases touch games then just throw the steam icon on your desktop and launch it. If you don't have touch games then you wouldn't be playing without a keyboard and mouse.

Lots of Steam games play well, sometimes even best, with a game controller.

The UI formerly known as Metro also seems that it would work reasonably well, at least better than WIMP, with a controller. Have anyone tried to plug in a controller to Surface, and see if it works?