What I mean is that 'boolean' ratings express an association, versus no association. Any item a user has bothered to rate is significantly more associated to that user -- even if they hated it -- than the universe of other items that the user has never even heard of.
If you're a classical music fan, and you dislike Brahms for some reason, I still think it'd be better to recommend another romantic composer before Led Zeppelin. So "dislike Brahms" is a more significant association than "no idea who Led Zeppelin is".
