When native XML databases appeared on the heels of the XML 1.0 recommendation, most people weren't sure what to make of them. Were they a replacement for relational databases or a return to hierarchical databases? No, said the vendors. They were designed to manage large numbers of XML documents. But since many XML documents could be mapped to relational tables, the only obvious targets were document-centric XML, which lacked structure, and catalogs, which had deep hierarchies. Beyond that, it wasn't clear what they could be used for.
The article continues at http://www.xml.com/pub/a/2005/03/30/native.html