It would be nice if Spring-WS did not only support SOAP, but also Plain Old XML messages. These are messages that do not contain a SOAP envelope, only a payload. The architecture is already adapter for this (i.e. most classes are SOAP-agnostic).
Note that POX is not the same as REST. In my opinion, REST applications can be made best with existing MVC-frameworks like Spring-MVC. Adding support for it would have no benefit (though the O/X mapping abstractions can be very useful in REST-solutions as well).