What if your XML is a document, that will be later converted to HTML, PDF an text? Then it makes sense to spell-check it the XML instead of one of the target formats. Granted your XML editor probably has a spell checker already, but if you use a pure text editor that has no spell checking capability (ed? ;--) to create short XML documents, or if you receive them from other authors that do not spell check them, then it might make sense to spell check the XML as a separate step.