In "enterprise" service-oriented architectures (SOAs), XML is a clearly entrenched format for server-to-server communications. But there's a movement afoot to define a "Web SOA" as a separate entity that describes server-to-client communications... based on JSON as the format.
The rationale is that JSON parses "a hundred times faster" in a web browser than does XML. This is like the tail wagging the dog. It is a myopic, accept-the-status-quo approach to web application computing, and it just won't do.
First of all, the difference between parsing different serializations is negligible (when done right), and nodes are nodes are nodes, so if there is a 100-fold difference, it is because one parsing method is direct and the other is doing something phenomenally suboptimal and probably easy to fix. One way to solve this is to alter all the system architectures on the web and introduce the complexity of a data format transformation to cross the illusory chasm between a "Web SOA" and the "Enterprise SOA". Another idea would be to put the pressure where it rightly belongs, on web browser makers to fix the feature of efficient XML processing.
Particularly as vertical industry standardization continues, such as is happening in both insurance and healthcare, this latter solution is really the only viable one in the long term. XForms technologies like Lotus Forms and Ubiquity XForms wrap a logical client application around the exact XML that is needed by the server side because this is the best way to minimize system development and maintenance costs. Once there is a defined XML data model, there should not be some other second data model introduced into web applications for some reason as vapid as poor implementation of the required feature.
This conversion is not a trifling matter either; have you seen how ugly the JSON is that is actually capable of preserving XML fidelity (i.e. supporting round-tripping)? Trying to build a reasonable client-side around that JSON is not nearly such a pretty sight as when the data representational limitations of JSON are accepted in an application. If the full representational power of XML schema is used to define an application data model, then we need to be able to carry that out to the client without significant alteration so that we can make references to the data transparently, not through the lens of some horrendous transformation. And make no mistake, XML schemas for vertical industry standards are full-featured indeed.
Finally, I would be interested in hearing about any research into whether the overall throughput of a web application is increased or decreased by the introduction of JSON down to the client. It's nice that parsing gets a hundred times faster on the client, but when you're server is trying to handle more than a hundred concurrent users, and the server has to translate from XML to JSON (and back again), then it sure seems like the use of JSON is pushing processing burden in the wrong direction.