Using an XML database (or using a storage/retrival mechanism that understands XML) is really useful if the domain of the data you need to handle is ill-defined.
in reply to XML for databases?!?! Is it just me or is the rest of the world nutz?
For example I just read an interesting article on FpML - a specification for data exchange for Over The Counter financial instruments between banks (swaps, FRAs, etc).
The problem here is that because these instruments aren't traded on an exchange the instruments aren't standardized, and have different behavior and characteristics depending on who the participants in the deal are.
With an XML-based system you can store the information regarding these instruments without having to re-define your database every other day because some smart trader has found a new way of doing a particular trade...
This doesn't solve all the problems of course - you still have to interpret the data correctly to perform appropriate accounting/trade reporting/confirmation/etc., but it at least enables the database to store the information without recoding it.