Is XmlUnit really necessary? NUnit makes sense since there is no inclusive way to verify code validity other than running it with known input. However XML was specifically designed to be verifiable using DTDs, and later extended to schemas. If your XML doesn't match the schema, it is't valid. period. So what does XmlUnit bring to the table that a validatingReader doesn't do?
It's not "necessary", but it's easier to test with it. It's just an abstraction on the kind of checks you and I would do anyhow with a validating reader or other things. Nothing wrong with making stuff easier, is there?