That method is only used for automatically signing users into the Admin interface, not to perform authentication on your public facing site. Meaning if a user is already signed into the Admin interface for another site and they switch to another site using the site switcher (upper left corner), that GUID is added to the URL and keeps them logged in to that site when they switch over.
This method is only used for the Admin UI purpose. If want to automatically log someone in based on URL parameters, I'd suggest looking into creating a temporary hash or something like that in the querystring and then requiring some other secret question to continue with their authentication.