I agree with you that putting JQuery in the browser is a bad idea, but the second part of the argument, that the browser will have the libraries in cache is not really that reliable. Here's some notes I collected on the subject: http://justinblank.com/notebooks/BrowserCacheEffectiveness.h....
I was definitely speaking whimsically because it is a huge chicken and egg problem. But theoretically if every site that used jQuery referenced a version at https://code.jquery.com/ there would be a very good hit ratio, even considering different versions. However we are a very, very far way away from that.
That may seem like a good solution for some sites, but the name of the page and the requestor's IP address and other information is also 'leaked' to jquery.com. This is not always welcomed. For example, a company has an acquisition tracking site (or other legal-related site) and the name of the targets are part of the page name (goobler, foxulus, etc.) which get sent as the referrer page and IP address to jquery.com or other third party sites/CDNs. While not a security threat, you may unwittingly be recommending an unwanted information leak.
