Web service description and modeling is metadata management. Let's not reinvent the wheel.
RDF has facilities to provide semantic transparency to descriptions and object models
RDF's ability to express unevenly structured graphs is a perfect adjunct to XML's similar data representation capability
RDF has proven ability to support a range of knowledge representation techniques
RDF has proven scalability (at least on order of relational database technology)
RDF integrates very well with most XML formats and technologies
RDF is reaching a critical mass of useful vocabularies, implementations, tools and practice
RDF supports more manageable XML data deployment by simplifying expression of inter-document relationships
Web services can take advantage of the general work in RDF query, rules: integration into general search engines