>Even in .NET JSON output is considerably slower than XML output for most scenarios, but speed isn't the only story - ease of use and easy compatibilty are the main reason JSON tends to be a better universal choice especially if you are exposing data publicly.
>

Agreed.
However, communication requires consent on both ends and most frequently the customer gets the privilege of specifying the interface.
Most have been specifying JSON lately but just today I'm working on an XML interface.
.NET has some excellent tools for handling XML.