It is not for the benefit of SOAP - it is for the benefit of HTTP which
is why it is expressed in a manner that is digestible by HTTP.
it's not for the benefit of HTTP, as HTTP already has a suitable dispatch
mechanism - namely, the request URI.
it *might* be for the benefit of vendors who want to sell upgraded HTTP
servers or client libraries that support the new feature, and force
customers to upgrade if they want to use SOAP. but I don't see that as
an adequate justification.
Your argument could equally well be applied to media types
media types aren't intended for dispatching on the server side.
however, the use of media types with HTTP applications brings up a very
similar issue - if the content-type is orthogonal to the request, and
the server can potentially handle the same request in different formats, fine.
but if the part of the request is communicated by the content-type, this
makes it impossible to transmit a properly-typed service request 'as data'
without implicitly requesting the semantics of that service.
Keith