Different sites might use different versions of a library. Also the cache gets polluted very quickly because many sites add caching headers for every request a

Different sites might use different versions of a library. Also the cache gets polluted very quickly because many sites add caching headers for every request and cache size is limited. If average page size is 500 Kb and jquery size is 30 Kb gzipped you do not save much by hosting is on a CDN. What you get is more DNS requests, more downtime when that server fails or stalls and give out data about your users. I think it is easier just to host everything on your own server.