Substitute "binary" for "XML" and see what you get.
"Using xml we can express/store *any data* in Xml."
"Using binary we can express/store *any data* in Binary."
"Binary is cross-platform. Any system can read and write binary."
"Binary is robust, and has been well-tested"
"Text data in binary is readable by humans"
Perhaps this is a good way to determine if XML is useful:
1. List all the reasons for using XML;
2. Use the "binary" trick;
3. Remove all statements that still make sense.
If your list is empty you don't need XML.
Cheers