[Miranda_]

Well, seeing as fan is short for "fanatic"; unless you really are fanatical about a band, you can't really call yourself a fan if you only like one song.

Having said that, I'm not really bothered about the term "casual fan"; it's the people who claim to be mega fans of a band when they're not who annoy me. If you claim to be a Manics fan, for example, you really ought to be able to name all the members.