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Database support

A popular feature of XML with protocol designers is that it is human readable.  It also can be used as a presentation format for database information.

There is a tension here between readability and bulk handling by machine.

In the context of a protocol, readability usually means that the protocol can be viewed and debugged using simple text-based tools.  XML works just fine at this level of readability.

There is also a tendency to use XML to encode other languages for which a significant level of ongoing human interaction can be envisaged (e.g. XML schema, XSLT, XPath).  My experience is that these languages tend to favour machine processability over human usability, and I would question whether that is an appopriate trade-off.