Complementing Open Source

Data is often more portable than code

Different kinds of promises Open source promises to let developers hack at code to make whatever improvements they need. Open data promises to let developers hack at information, and even take that information elsewhere if they can't make it work with given programs.
Interop with closed source Open data doesn't require an open source model, though it works well with open source. Closed source tools can process, generate, and store XML as well, making XML a potential bridge between open- and closed-source projects.
Data hijacking possibilities XML's hype has driven lots of companies - notably Microsoft - to use XML formats in their products and infrastructures. This means that open source projects may be able to hitch a ride or even divert traffic from such projects.
Extending the open source philosophy While nothing about XML demands open source development, there are a lot of opportunities inherent in XML which mesh well with open-source development.