While I don't know anything about fonts stuff, successful execution of the ad tag leads to an ad request, and this happens on every page view which has the script and not just the ones where the file has fallen out of cache. So there's no additional useful information Google would get from a low cache lifetime for the script.
The script request is to a cookiless domain for performance, unlike the ad request, so there's not even much useful information on that request.
(Disclosure: I work at Google, on ads JS, and I previously worked on mod_pagespeed. Not speaking for Google, just myself.)
The script request is to a cookiless domain for performance, unlike the ad request, so there's not even much useful information on that request.
(Disclosure: I work at Google, on ads JS, and I previously worked on mod_pagespeed. Not speaking for Google, just myself.)