What a RelaxNG schema gives you is the ability to use a schema aware editor to create the class and avoids needing to create checks and create error messages for invalid ones. Also you have a formal description of the structure of the configuration file, rather than an informal one in the form of your code and documentation.
Of course, I'm only saying this cause it's new and shiny to me and everything looks like a nail at the moment
For what it's worth, the program already uses a domain specific markup language at the moment ... "Name:" is markup.