It's not about indexing paths you indicate, XML databases are about indexing the whole structure (remember, XML is not relational, but are graphs). Besides that they provide XPath/XQuery processing and optimization. You can query large XML documents or sets of documents, like you'd query them with e.g. XQilla.

Of course, it is possible to implement all of this on top of existing database technology. E.g. Oracle's Berkeley DB XML is implemented on top of Berkeley DB. But, a relational database with some indexing of XML does not provide the same functionality as an XML database.