Where something like JSON or (if you must) XML is used I prefer the idea of REQUIRING the preservation of all fields in the original structure UNLESS a field or

Where something like JSON or (if you must) XML is used I prefer the idea of REQUIRING the preservation of all fields in the original structure UNLESS a field or set of fields is validated and then updated in place. This lets end users and extension authors do things that make sense, such as adding a comment tag ( JSON added key:value "__customcomment": "Any well formed string will be safe here." or XML <!-- Please don't eat me when parsing the config! --> ) to a configuration file stored in either format, and actually having it persist.