In general it is nothing else then introducing a new datatype of type "table". But there are quite some implications. If this is a table, one for sure wants to select from it, possibly with a "where" clause or other operations.
So what most vendors do is first decoding the XML into a set of tables so their tool can deal with it. The disadvantage comes if a XML should be copied or at least the structure remains pretty much the same. In the case the engine has to decode the XML into tables, perform the operations and then rearrange all tables back to XML. These are heavy operations although almost nothing has to be done.
For this reason with Data Integrator we were going the hard way and implemented full support.
But still, what is it good for? Well, first, if you deal with XML reading and/or writing NRDM capabilities are necessary. And keep in mind, XML is used in many different areas: WebServices, JMS Messaging, MQ messages,... But the same things happen if you call a stored procedure that returns an array. You get one result row, but one of the columns is of datatype "table" - the array. Especially in the SAP area this functionality is very important.