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.
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Do a significant percentage of users spoof their agent?
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.
It spoofs the platform, not the browser. Mobile Chrome/Safari should have feature parity with their computer cousins.