Is this usually actually true? It makes a ton of obvious sense, but I've seen numbers thrown around that suggest a surprisingly low number of people actually end up taking advantage of the cache.
It's 100% not true. It's also a pain in the arse if you serve everything over SSL like we do.
To the best of my knowledge, the only reason SSL could cause a problem is if you are specifying the protocol (http:// vs https://). The best way to handle this is to use a protocol-relative URL like "//ajax.googleapis.com/mylibrary". The browser will then infer the correct protocol.
You've obviously never dealt with ancient IE versions and script loading then...
I'm happy to be enlightened if you feel like sharing.