You're uniquely positioned to comment on this, of course. Are there other music notation formats that do a better job (MEI, MNX, etc.) ?

I've been working with MusicXML as a consumer (and occasional contributor) in my own apps for a few years now, and my pet theory is that the _problem domain_ is so complex that any format will fall short of the universe of use cases - or become so cumbersome that its application to a codebase will forever remain buggy / incomplete.