I don't think anyone will be replacing their enterprise datbases with XML files anytime soon. I think XML is "universal data" when talking about the need for data
exchange between different systems or different companies with different systems.
Meaning that you might be using Oracle and I might be using Informix, and we decide we need to exchange some data with each other through an automated process (HTTP, FTP, etc). If we agree on a set of tags to use in an XML file, we each can develop software in our language of choice that will create or process the document based on our particular database platform and schema. That way, even if I change over to SQL Server, the way you and I exchange data does not change.