Jeff Atwood recently blogged about XML as "The Bracket Tax".  Jeff usually has some great observations, however labeling XML as a "Bracket Tax" is a bit rediculous. 

"Everywhere I look, programmers and programming tools seem to have standardized on XML. Configuration files, build scripts, local data storage, code comments, project files, you name it -- if it's stored in a text file and needs to be retrieved and parsed, it's probably XML. I realize that we have to use something to represent reasonably human readable data stored in a text file, but XML sometimes feels an awful lot like using an enormous sledgehammer to drive common household nails."

The reason why people use XML is because its a proven standard.  Every modern language has an XML parser.   XML is not used because its "efficient", it's used because its a farily safe bet that most developers will know it and also because its pretty widely supported... Almost every language and framework has an XML parser.    

XML as a language is more than just angle brackets and self describing data, XML offers extended functionality such as XPATH for querying, XSD for schema design and validation, and let's not forget the mighty XSLT for transforming documents.

Contrary to some opinions, XML is a very successful language.  Efficency zealots can go ahead and use SSYN for passing data, ConfigObj for their ini files...  But think about who will eventually be maintaining your code... you think they'll know JSON, SSYN?  Maybe... but my money says thay will know xml.

**Update 6/25/08**
Apparently I wasn't the only person who disagreed with Jeffs post.  He has posted a clarification to his position.  View it here.