For me JSON's strength is that you can throw typed data in simple structures straight at it and be confident of getting the same out the other end without worry

For me JSON's strength is that you can throw typed data in simple structures straight at it and be confident of getting the same out the other end without worrying about the exact details of the encoding. If you exceed the supported types you have to work harder but in great many cases that is not necessary. If MsgPack doesn't let me throw strings, numbers plus arrays and dictionaries containing arbitrary further structures without me having to worry about defining the encoding it isn't on par. Obviously any sane data format can contain any data but not equally easily or efficiently.