Lessons: Use Cases matter, and programmers (the users in this case) will choose tools that are both simple, in that they are not complicated/over-engineered, and easy to use, requiring little setup and code to accomplish a task. For parsing data with PHP, contrast using something like SimpleXML or DOMDocument (which is light-years better than where we were in parsing XML just 5 years ago), to just doing json_encode() or json_decode().

In particular, JSON shines as a programming language-independent representation of typical programming language data structures. This is an incredibly important use case and it would be hard to overstate how appallingly bad XML is for this.

James Clark's Random Thoughts: XML vs the Web