Most mobile browsers have a "request desktop site" feature, which I assume would involve spoofing the user agent to look like the desktop version.
Most mobile web applications that would use this polyfill wouldn't have a separate mobile / desktop site and have just a good responsive design, so mobile users wouldn't feel a need to request the desktop site. That feature is usually used (I assume) because the mobile versions of websites often offer limited functionality.
Do a significant percentage of users spoof their agent?