Kim Cameron pointed to my questions about WS-Mex and Don Box gave his reasons for why WS-Mex is needed over and above simple HTTP. I think it basically comes down to the fact that SOAP and its related protocols are, ideally, transport neutral. WS-Mex is one component of that transport neutrality. In fact, in doc/literal mode SOAP is really nothing more than transport anyway. Like many things in Computer Science, SOAP is simply another layer of indirection and the costs and benefits of indirection layers are well understood. WS-Mex is one way of preserving the SOAP-as-transport indirection.