With reference to my earliest blog post on "Relational/XML hybrid database design",
I have some more thoughts on this topic.
I think, an information model of an enterprise should consider XML as a first class data storage mechanism, along side the relational mechanism.
When XML was not there, using relational storage (and some other forms of EIS data) was the only choice. Given the benefits of XML (some of which are mentioned in my previous blog post cited above), it should be considered equally important as the relational mechanism, for data persistence needs.
There should be a mixture of XML and relational data in the overall data model of an enterprise. This would bring best of the worlds of relational and XML storage, to the enterprise's data.
I have seen that, today XML is used a lot for various kinds of property/configuration files, and for sending messages. But I feel, that wherever it makes good business sense, the core business entities should be modeled as XML along side the relational models. With the current hybrid XML/Relational databases (like, DB2 with pureXML), we can have the relational and XML data in the same transactional context.