It depends on what your "plain old XML" contains.
The fact is that POX was compared with SOAP. SOAP is a very specific way of processing XML (mostly) HTTP, and people have already run "plain old XML" through HTTP. SOAP received a lot of criticism because it complicated what people already did with POX. But I will not delve into this, I am trying to say that people used POX for RPC .
REST RPC. REST XML , , RPC. REST, , , , XML: HATEOAS.
, POX REST, : , REST.