[olavk]

Of course you have to negotiate the terms of the transfer. You have to do that using any format (custom binary, XML, JSON, s-expressions, whatever). XML just defines a lot of common syntactic stuff which you would have to define anyway in any format you decide on.

[sah]

I would argue that there's very little value in standardizing that syntactic stuff. Whatever tiny amount of value there might be is probably destroyed by picking a convention as almost universally inappropriate as XML.