One of the biggest underlying benefits of using a shared, public CDN for this is that you can take advantage of cross-site caching. As more and more people use

One of the biggest underlying benefits of using a shared, public CDN for this is that you can take advantage of cross-site caching. As more and more people use it, the potential for that is greater and greater. However, it only works if sites are referencing exactly the same URL; just referencing the same file is unfortunately not good enough. So, using private CDNs like those don't confer quite the same benefit (though they're a great idea for hosting site-specific assets, of course).