If you’re only using XML for AJAX style communication of fragments of data between a model and view, sure, use JSON or whatever. If, on the other hand, you’re encoding a representation of a text and need to mark it up to indicate bibliographic features and textual variation between editions, you need TEI XML. If you want a dataset to be available in a machine-readable format at a particular address and also have that data look good for humans, you want XML. XML is for more than passing messages from a backend to a frontend.

I wasn't meaning to imply that XML is only for backend to frontend communication. I only mentioned they as it seems to be the most common task in web jobs today and is usually what a person does most often when I hear the complaint that XML is pointless and we should just use JSON.