> Also the point about the ad-hoc client validation doesn't seem right to me. If the server changes the XML response, the client may be able to fetch the update

> Also the point about the ad-hoc client validation doesn't seem right to me. If the server changes the XML response, the client may be able to fetch the updated schema (assuming the devs didn't forget to publish the update...), but then what? You still have to handle the new or updated fields in code somehow. To be fair, there are often tools that will generate object definitions in the target language from an XML schema. It's not a perfect solution - you need to rebuild the project when upstream changes - but it's not like this hasn't been addressed. For what it's worth, Protobuf has the same property.