"Walk in to any library on the Dewey system and you'll find Beethoven shelved under Classical."

Well that's right. You'd also find Palestrina, Gesualdo, plainchant and Hildegard of Bingen, as well as Michael Tippett, Britten and even Gershwin under the same category. Maybe even Harrison Birtwistle. When I did a course on plant taxonomy at university we were taught that classification was there to serve several purposes, and the same body of plants could be categorised in a variety of different ways according to to the user's need. No one system of classifying music can be set in stone as the be-all and end-all. There's a limit as to how much you can atomise the classification of music genres before you make your system useless to library users. On the other hand, a scholarly analysis of the last thousand years of "classical music" will break it up into much smaller categories, a classification system which, among other things, helps to elucidate the evolution of music down the centuries. When I walk into the library I want big categories to direct me quickly to the right ballpark of bookshelves. After that, even alphabetical might cut it...