I came across another piece of software which uses XML-RPC.NET today: Xindice.NET. It provides a .NET interface to Apache Xindice (pronounced zeen-dee-chay) via its XML-RPC interface. Apache Xindice is described as:
The Xindice Core server is a database server designed from the ground up to store XML data. The Xindice server is what is termed by the XML:DB Initiative as a Native XML Database. You could also refer to it as a seamless XML database which might be an easier to understand description.
This is all new to me but this DevX article makes things a little clearer. XPath is used to query a Xindice database and XML:DB XUpdate is used to update XML contained in a database, updating specific elements of an XML document without rewriting the whole document.