"Unfortunately, public identifiers do not fit neatly into the existing web architecture because they are not legal URIs. Many new specifications (XSLT, XML Schema, etc.) have the implicit or explicit requirement that all external identifiers be URIs. The purpose of this namespace is to allow public identifiers to be encoded in URNs in a reliable, comparable way." see also
Norman Walsh on XML Catalogs and
Why URLs are good URIs, and why they are not