I suggested this because I believe that if a DTD is well-written, it can be validated by an external program and the config file can be validated against the DTD by an external program. They can also be validated by the app itself, but that may be undesirable. Some issues might be: long startup time to discover a bad config file, a poorly written app that does not check all of the config file before using it and dies into the running when errors are encountered.
For me, though, I like the idea of being able to tell a running program (e.g. via a button or signal) to re-read a config file to cause changes to occur. It is nice if the file is externally validated before it is reread. Not that everyone has such a need, but in the development phase, it is sometimes nice. Also, I like being able to have an external description of the config file (a DTD) that I can keep in one place on a network that many apps/validators can share. It sure makes maintenance easy. (Yes, one could do that with other formats, but it is "built in" to the whole DTD concept.) I also like the idea of the whole config file being sucked into the app as a large data structure I can access "easily".
Anyway, I would like to know what others think about configuration files.