One thing that's nice about XML (versus CSV) is that it lends itself to hierarchical data. CSV works best for 2 dimensional data (i.e. rows and columns). So for example, when MobileSheets is updated to allow multiple artists for a song, this might be messy to handle in a CSV file (i.e. a semicolon separated list?), but it would be easy in XML. (I am probably preaching to the choir [ha!] for our program author)
Oh, one more note: if you have a recent version of Excel, it can load and save XML data (after you create an XML map), so an existing CSV file can easily be converted to XML.
Oh, one more note: if you have a recent version of Excel, it can load and save XML data (after you create an XML map), so an existing CSV file can easily be converted to XML.