I have a site that logs AJAX requests to Google Analytics (since that's essentially the only "page views" on the particular site). I was doing a test for if (_gaq) before trying to push into the array, because I wasn't more concerned with giving the users a good experience than tracking page views, and that did work if Google Analytics was being blocked from loading at all. Unfortunately, I eventually learned that Disconnect was causing that test to throw a JavaScript exception instead of just blocking GA from loading and leaving _gaq undefined. As you can imagine, narrowing down that it was Disconnect causing the problem from a few random users' "the site isn't working" complaints was a lot of fun...
It's not a huge site, but I do spend a couple thousand dollars a year keeping it online. I'll be damned if I'm going to spend that to keep a free site online and then feel bad about running some advertising and analytics to make sure it doesn't end up in the red.