Disclaimer: I have no experience with MusicXML. The vast majority of the music I want to notate fits on a single staff, and ABC is brilliant for that -- it's a well-established portable open music format that's easily entered and edited by hand. (Technically speaking it can do multi-staff music as well, but it starts to get a bit over-complicated and hacky at that point, IMO.) http://en.wikipedia.org/wiki/ABC_notation

PS The "Score" example on the ABC notation page is terrible, but it's a limitation of the lousy standard abc2ly translator, not the ABC format. I've got my own abc2ly which does a much better job of it...