I know that one of the reasons that Apple went with XML for launchd was that you can define an XML schema for the files and validate them against that schema. IIRC, this claim was made in a HN (or Slashdot) discussion maybe 5 or 6 years ago.
Honestly, I think the statement, 'XML should NEVER be used, period,' is true as well: for any use case I can imagine there are superior alternatives to XML. Even if you have to interface with other services using XML, just use SXML (https://en.wikipedia.org/wiki/SXML) in your own code and marshal to XML at the very last instant.
Ultimately, XML was a colossal mistake carried out to perfection.
SGML and XML were invented as a meta-language for encoding semistructured text such as HTML with plain text editor programs, and excels in that use case like nothing else. That XML is abused as config language, payload meta-syntax for web services, for encoding component models, and even as meta-syntax for programming languages is hardly SGML/XML's fault.
HTML itself allows SGML-style tag omission since HTML was originally an application of SGML. A simple example can be found at [1], and can also be seen in action in my talk slides linked from [2].
We're talking XML rather than HTML (and omitting certain closing tags isn't compliant HTML5 anyway).
In XML there's also the concept of using self-closing tags only for "empty tags". Meaning, <tagname val="123"/> isn't "correct" and <tagname>123</tagname> should be used instead; while s-expressions simplify this.
Sorry but your comment re XML is incorrect. I suggest you study the HTML and XML specs, especially if want to convince us of an alternate XML serialization.
It hard to take a hardline stance on something with so much give and take.
XML is great because there are plenty of fast parsers for it with bindings in pretty every programming. It can be modified with nothing more than a text editor, by someone halfway competent.
XML is bad because it will never be quite as optimal as some binary only solution and editing with a test editor is painful.
Its not like most programming languages would have a hard time with gzipped xml either. This grants most of the benefits of binary formats and is often smaller than all but the most carefully designed ones.
All of that even presumes config load time matters. I a crazy large case maybe 10MB loads from a disk and the important stuff is cached in memory, in the actual structures that will use the data later. It just doesn't matter. Gzipped xml is fast enough for realtime high performance games, who cares about this silly XML hating anymore. Just pick something that works and move on.
