Isn't this just copy-paste for 5 dollars?
This isn't a general rule of thumb, sometimes having your code inlined can be more beneficial especially if you are on an average/high-speed network where downloading 1 - 5 megabyte file can be a lot faster than using CDNs also, in both cases the browser will try to cache the page so subsequent loads are faster. If you're building a single-page app having everything inlined and delivered as a single HTML file can be much easier than distributing multiple files.