I'm not saying it's a bad thing. I'm merely trying to correct the perception that it's a storage methodology. The particular container is the methodology. What's in the container might be XML or plain text or any number of things. The container might be a flat file or a relational database or something else. One chooses the format for good reasons and one chooses the container for (other) good reasons.
Yes...Databases are built in such a way that changes to the structure does not effect any of the program used to access the databases.