In addition to Cache Partitioning, it was never really likely that a user had visited another site that used the same specific versions from the same cdn as your site uses.

Making sure all of your pages were synchronized with the same versions and bundling into appropriate bits for sharing makes sense, and then you may as well serve it from your own domain. I think serving from your www server is fine now, but back in the day there were benefits to having a different hostname for static resources and maybe it still applies (I'm not as deep into web stuff anymore, thank goodness).