Parts is Parts

Once the doors open... If the data formats used to exchange information between processes are open, you can modify data flows and create your own data sources and destinations. Software architectures, not just markup, become Extensible.
Simple things can do real work Exposing data in XML makes it possible to hand off tasks that may be simple, but can be repetitive or laborious. The cost of serializing and reparsing a long list of names may be far less than the cost of sorting it, which can be handed off to another system.
APIs exposed as well While XML documents are really handy, passing SAX events or especially DOM trees within an environment is just about as easy, and doesn't involve the same overhead. This can make it easier to partition logic.