Thankfully, there has been some movement towards supporting the Steam Controller outside of the Steam app, as the SDL (Simple DirectMedia Layer) library now supports the controller's "touchpads, capacitive touch for sticks, and grip sense handling" (via Phoronix). In theory, that means certain games will begin to recognize the Steam Controller as a fully-functional joypad when you play them on PC. You might not see SDL support across every AAA title, but it's a major step towards controller functionality on Windows when importing non-Steam games. Testing the current functionality myself by add