The problem discussed are that XML is somewhat complicated to parse correctly. If you only support a subset of XML then it is easy to parse. The problem is that I might not know what subset you are using. When I send you advanced XML (since you said you support XML) something might crash. There is probably a need for a simpler subset of XML with its own name (like XML 2.0 or XML-WS or Mini-XML). Personally I would like to skip the requirement for a root element. Since json is a bit simpler it has less of these problems. Of course it has other problems but that is another thing.