Recent quotes:

Why ISBN?

what becomes obvious in any discussion of the ISBN and/or other identifiers is that there are two interests, sometimes running parallel to each other. One interest is in discoverability, and many authors say they see no benefit from the ISBN in that regard. The other is the quantification and understanding of the size and scope of publishing, which can be accomplished only by counting industry output -- which cannot be done without some form of identifier, a tag.