LW uses that domain to serve jQuery and Prototype. This is a recommended practice. The voting and posting code both rely on these libraries.
It sounds like what we want is failover: "if the user can't get jQuery from Google, then give it to them from LessWrong." Here is how to do it.
CDN failover is a best practice in general because it keeps the site working if Google ever goes down.
Apparently, by not unblocking scripts for "ajax.googleapis.com", I am unable to vote on LW. I generally dislike enabling scripting for domains that are used in many places -- unblocking Google APIs would unblock it everywhere, not just here -- so the result is that I am no longer voting. I suspect that I am not alone in this.
(Apparently I can't post without enabling it either. Looks like I'll have make an exception and do the script-on-script-off dance after all. Whee.)