Ah ok. But to assemble XML/SOAP for these kind of payloads you'd typically be using a lib such as JAXB, Jackson/gson, Spring (for Java at least) or SCA anyway.

Ah ok. But to assemble XML/SOAP for these kind of payloads you'd typically be using a lib such as JAXB, Jackson/gson, Spring (for Java at least) or SCA anyway. Moreover, you could design your XML Schema such that both variants are accepted (using XSD "all-groups"), though typically record-like data structures are indeed described using XSD sequences which would cause the issue you mentioned. Again I agree XML/XSD isn't a great payload/schema mechanism for data structures such as maps; after all SGML/XML was invented for encoding text. It's being used because it was the first mechanism on the scene, and it continues to be used because it's still much more capable and mature/established in terms of data validation than anything JSON, especially for protocols used in communication with external partners/banks/authorities.