I think you have missed one of the main benefits of XML. Its not an ideal storage medium, its not a lean storage medium either...

Where it does excel is in transportability. We have a very major data store for the travel industry which we offer to customers via a XML interface. The reason we use an XML interface is that it is the only way we could easily distribute the content in a standard format across many channels.

The data is a complex relational set and could not be distributed (In real time) any other way. We couldn't pass a series of CSV files for the related data, XML was the only way to go.

XML is fantastic at what it does and has opened up a whole new area of data management and distribution for many companies, system can be developed that were just not feasible before.

We do however store and manage our data in a fully relational SQL database.

Dan