XML isn't painful because it has a schema, XML is painful because it wasn't really designed for RPC, so getting to feature parity with something like Protocol Buffers takes a whole stack of XML technologies and a huge mess of complexity.

Protocol Buffers were designed from the ground up for RPC, and as a result are far simpler and more convenient to use than XML. Seriously, nobody who uses Protocol Buffers compares them to XML, because it's not even a comparison.

https://developers.google.com/protocol-buffers/docs/overview...