When Google switched to AJAX-based search, that temporarily stopped sending referrers, and lots of people screamed bloody murder. For example, http://getclicky.com/blog/150/googles-new-ajax-powered-searc... said "So what can we do about it? If you run a blog, write about this. Submit this story or your own story to large tech blogs like TechCrunch, CenterNetworks, ReadWriteWeb, GigaOm, etc - no large site has written about this yet, and one of them needs to. ... Do anything you can to spread the word and let Google know that this is not acceptable."
Or see http://www.seobook.com/Ad-Networks-Partners-Hoarding-Publish... or http://econsultancy.com/us/blog/3240-google-ajax-bad-news-fo... or http://blogs.sitepoint.com/2009/02/04/google-update-breaks-t... . So we heard lots of complaints.
SSL as an option provides a nice choice for people who care about these issues and don't mind taking a tiny hit in latency.