Hosting ga.js locally has two main benefits – fewer DNS lookups for the client’s browser (you could even combine it with your site’s java script to reduce HTTP reqs as well), and you might be able to get it to your users faster than Google can. Whilst the second point isn’t likely true for many, it is true for those of us serving most of our content in Australia.

Google recommends you don’t do this, but doesn’t say you can’t.
» Read more: Hosting ga.js locally (google analytics tracking code)