Sample use case: You have a .war in an .ear, and the .war has a web.xml with a <servlet-class> that refers to a JAX-WS endpoint, so the class is annotated with @WebService. If that class is packaged in a jar found in the ear's lib directory, the JBossWS DUP won't pick it up, and then Undertow will throw an error like this (except of course the JBossWeb equivalent):

UT010009: Servlet ClientEndpoint of type class com.redhat.gss.jaxws.ClientEndpoint does not implement javax.servlet.Servlet