I imagine a lot of other people share your sentiment because I haven't seen an XML database worth using (yet), but I hope that there will be one in the future. If you don't have a reason to use an XML database, don't. That said, a few reasons come to mind why you might want to use one.
in reply to XML for databases?!?! Is it just me or is the rest of the world nutz?
1. You get the information in XML (perhaps via SOAP), and you use the XML (perhaps with XSLT) for your application, and you need to store it. In this case, you save the trouble of converting it twice (XML->RDBMS->XML).
2. You have no control over the data you are receiving. Today, you might get data for a, b, and c, but tomorrow it might have a, c, x, y, and z. Also, you need to store that additional information in a way you can "easily" use.
3. Your application is simple, but the data is complex (yet easily fits into XML). Why spend a lot of time creating a complex RDBMS scheme when an XML database could handle the job?
Also note that (in my experience), reduced development effort is usually worth additional overhead. Even though this is obvious, I'll say it anyway: you have to weigh the trade-off between system overhead and development effort in light of specific project requirements. If we were stricly concerned with system overhead, we wouldn't be using perl at all... or XML :)