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Greg Reinacker: For cases like these (and many more that I could come up with), I believe that loosely relying on the schema is a bad idea. I think you should only extend the request in places where the schema specifically allows it; in the case above, if there was a xml:any extensibility element, you'd be fine, and you could put your extensions there. If you want to omit parameters, you can only omit the ones allowed by the schema.
FYI: Not mentioned in the essay: ASP.NET omits parameters who's value is null, despite the schema it returns in a ?wsdl call - for both doc/lit and rpc/enc versions. Hence, the client depends on the behavior which is documented in the essay. Meanwhile, try defining a method in an .asmx file with n arguments, generate a client, and then modify the .asmx to have n-1 arguments.