The format seems to address the alignment of words and melody for a printed output. Presumably the source XML file can be re-edited if the tune needs to be modified. Looking at examples of a score compared with the XML file, it looks as though it codes each vertical "slice" of the stave as a separate entity, so could be difficult (although possible) to edit by hand.
It is designed to be produced and read by purpose built programmes, although if the format is open source, there may be free progs which will do it. If it is becoming a format supported by a range of music editing programmes, it would definitely be worth investigating.
Here is a page of links to rogrammes which can use Music XML BLICKY.
This one musicrain seems to be an elegant solution for displaying notation, words and audible tune online, as long as you have a browser running recent version of Flash Player. Look at the Demos - transposable and printable score which also play as music. I couldn't find any mention of how much it costs though.
Quack!
GtD.