Someone in another thread (sorry, don't remember which) made a good point that including several frameworks by default would discourage people from trying new/novel/less popular frameworks.
I think linking to the library from Google's CDN is a nice compromise since it greatly increases the chance that it'll already be cached.
You can always link to the framework, so at worst your user will find the site a few seconds slower to load.
Then if there is adoption, you can always be included in the next build of the browser. All these frameworks tend to be tiny(i.e. jquery is huge, but is something like 30 kb), so nothing is stopping the browser from including all the possible frameworks on the harddrive.
I think linking to the library from Google's CDN is a nice compromise since it greatly increases the chance that it'll already be cached.