See Special:Export and Special:Import. I'd like to transition our public backups to this format at some point.
A non-standardized undocumented XML format is not really much better than the database; the purpose of the exercise is standardization, for now and the long term. Also, It needs to deal with such issues as properly recording the context of the data, the user database (with its attendant privacy issues), standardization of the wiki syntax itself, and other issues.