Why would you output XML 
The Wikitax aims at being as simple as possible to write. But if you want to reuse text of or for the Wikipedia you need to write a filter for each format. As far as I know the Wiki system only provides a conversion from Wikitax to HTML.
If there is an XML syntax for Wikitax and a convertion between both of them then:
- You "only" need to write an XSLT filter to import/export other formats.
- A PDF view of articles (with FO)
- Editing articles in OpenOffice
- A Wikipedia Client using OpenOffice as GUI? (Wikitax may be simple but people prefer WYSIWYG!)
- How about a wysiwyg editor that doesn't take over a minute to load? ;)
Other side effects/benefits include 
- You could use another wiki syntax to write articles for wikipedia. Some people might like to write *foo* or <b>foo</b> or '''foo''' but it's all the same, isn't it?
- There is a discussion on a Wiki:WikiInterchangeFormat (an open and standard data format for sharing wiki pages) that may be XML. A group of us (representing MediaWiki, TWiki, and PurpleWiki) gathered at Wikimania on August 4, 2005 to discuss ways of pushing forward on a Wiki Interchange Format.
There will probably never be one XML format for all wikis. For the Wikipedia project we should define a format that is convertable to Wikitax and vice versa. every other converting tasks may work on top of this.