While tools for writing XML are not as developed as those for SGML, there are a few reasons why you may want to start writing in XML:
Libraries for handling XML files are developing at a rapid pace. These utilities may make it easier for new authoring tools to become available.
Many popular word processing programs are now creating XML output. While it may not be DocBook XML, this does make it easier for application writers to either add DocBook XML support, or provide some method of translating between their format and DocBook XML.
Everyone else is doing it. While this might not be a real reason, it allows the BDP to keep up-to-date with similar projects. Tools, procedures, and issues can be worked out in a common framework.