actually, XML is a markup language just as much as it is a data format language. an eXtensible Markup Language even. the tricky part is that XML has no semantics of its own. it is just syntax. this may have been what you were trying to say, but it wasn't clear to me.
the fact that you can markup a document with XML makes it fundamentally different from just a fancy version of CSV. i would even go so far as to say that XML's ability to be used for marking up documents is much more important than as a data format. if you're already exchanging data in CSV files, there isn't much of a case to be made for switching to XML. but if you are trying to exchange any kind of document there is likely to be a reasonable case for converting to an XML format.