A few days after Norm’s post, two other posts appeared refuting his stance even though in many comments it is considered a non-debate. First, Manu Sporny talked about the move to JSON being more of a paradigm shift to simpler markup. The complaints about SOAP and XML Schemas are obvious, but he complicates the argument by introducing JSON-LD into the conversation. JSON-LD introduces syntax for JSON to denote LinkedData, and there is notation very similar to XML Namespaces, to which Norm replies “Wow. By the time you start doing that, you’re sure you wouldn’t be better with a richer markup vocabulary?” Lastly, James Clark throws his opinion into the mix. His commentary is more about the fact that XML is losing web developers which could be a bad thing.
However, what if you are using PHP or Java on the server? PHP has plenty of XML handling libraries, with SimplePie being a hugely popular RSS feed processing library. If you can make the Twitter API call from your Java server code, there are plenty of libraries for handling XML there as well. So, in that context XML may be a better option.
As with any programming problem, different requirements and different contexts may call for different technologies. If you get stuck on saying that JSON is better than XML (or the other way around), you lose another tool in your toolbox.
The other context that people are missing is why Twitter and Foursquare chose to support JSON only. This is likely a question of application complexity and analytics. Like any good API provider, Twitter is probably tracking all calls to the API and this includes the data format requested. It is very possible that the demand for XML was fairly low and it did not warrant separate support. In addition to this, there are plenty of JSON processing libraries available for mainstream languages like Java, so there was little risk in dropping support for XML. If there is no support for XML, then their API becomes simpler to support. That means less code to maintain, simpler maintenance of code because there are not multiple representations of one set of data, and fewer questions about the different formats.
So, quit whining about whose data format is better. Each one is better in a different context, otherwise it is highly unlikely that they would have become so popular. The important thing is to learn both formats, and other popular ones that appear, that way you can make an educated decision on which format to use in your situation.