No doubt Google considers it a best practice, because they are in the business of knowing what people do on the internet. How often will a browser get the jquery for a particular website or check with the CDN? There are a lot of jquery versions in use, the cache gets deleted (often on browser close) or expires, cache-age=0, etc. Almost always?

Besides providing jquery themselves, websites can use a CDN from another provider. OK, that might cost an extra 20 ms.