In practice, this never happens. The value is certainly less than the data you're selling about your users to Google.
Edit: here's the ToS: https://developers.google.com/speed/libraries/terms If you believe Google, they're not collecting anything.
My tests show local resources consistently loading faster than making a link to Google. These are supposed to be cached but thing like fonts are always loaded again. Anyone can repeat this for themselves.
I tried, they came from the disk cache.