I'm not a DW expert but this has been my experience so far.
XML give you the ability to create a universal format for data exchange between applications.
In the DW world there is something called XMI, which is XML based, that provides a standard way to exchange information between DW applications i.e. tools.
The benefit I have experienced from implementing XMI is:
1) my ETL tool is now fed from the ODS, modeling, and cleansing tool.
2) the reliability and accuracy of the data going to DW and DM have greatly improved.
Hope this helps.