That idea of having JS files hosted elsewhere always struck me as a Girardian scam (e.g. "everybody else does it") and always getting voted down when I showed people the reality factor.
Nobody seemed to think it was hard to host a file before this came along, just as nobody thought it was hard to have a blog before Medium.
Of course this creates the apocalyptic possibility that one of these servers could get hacked (later addressed with some signing) but it's also not easy to say you're really improving the performance of something if there is any possibility you'll need to do an additional DNS lookup -- one of the greatest "long tails" in performance. You might improve median performance, but people don't 'experience' median performance in most cases (it goes by too fast for them to actually experience it), they 'experience' the 5% of requests that are the 95% worst, and if they make 100 requests to do a task, 5 of them will go bad.
People are miseducated to think caching is always a slam dunk and sometimes it is but often it is more nuanced, something you see in CPU design where you might "build the best system you can that doesn't cache" (and doesn't have the complexity, power and transistor count from the cache -- like Atmel AVR8) to quite a bit of tradeoff when it comes to 'computing power' vs 'electrical power' and also multiple cores that see a consistent or not view of memory.