-Perfectly readable to me, works fine with auto-collapse. Closing tag is complained about but it helps you keep track of what you're reading.

-Cleaner than "simpler" serialization formats in many cases, like putting type information in an attribute instead of something hacky like giving your json object a $type member. Ran into this myself serializing objects in an array of BaseClass where the class of the objects mattered, had to turn on a flag in the JSON library.

-Extensible when you need it, as the X stands for. As in the prior case, mapping extensions onto more rudimentary formats becomes hacky and less readable.

Obviously it's going to be easier to use JSON et al in some contexts but I never felt anything was inherently wrong with XML. XML is still probably more appropriate if you're storing a lot of metadata to your objects and you're not in a context like web dev where only other formats are treated as first-class.

There are some things I don't like about JSON. It's supposed to be easy for JS users because it's literally "JavaScript Object Notation" but it has annoying differences like requiring quotation marks around variable names and disallowing // comments. Not intuitive at all.