Currently, I have used many different methods to bring music in/out of BIAB/RB, including everything from MIDI to parsing the raw MGx files. The thing that very few people on these forums seem to consider are the publication-related problems that someone who must "describe" music to another person suffers.

There are many reasons for wanting symbolic representation of the music, such as Fake Books for Jazz and Guitar Tabs for Gypsy Jazz, rather than wanting say an MP3 or MIDI file that performs the music.

Granted, BIAB does an "OK" job of printing PDFs, and by hacking around, you can even generate things like Box-Charts. However, after working with other music publication systems, I don't think of BIAB as a fast and flexible music publication system. For example, a complex song can fill a whole page with just the box-chart chords, but how do you create a second sheet that has just the guitar/mando chord charts, including fingering for the chords. I don't mean a tiny chord diagram for each chord on each bar of the song - once someone knows how to finger the chords, all you need is the box-chart with the chord names. Plus, BIAB only has chord libraries for Folk and Jazz guitar chords, for example, so things like accurate accompanyment for Gypsy Jazz is impossible to insert into lead sheets and chord charts.

Creating Tabs is also something that needs a pro music publication system, plus a good app like GuitarPro, that can pull in a BIAB file, create the actual chord diagrams to be used, and add the real Tab symbols so that a person who reads only Tabs knows about hammer-ons, slides, tremolo, etc.

Because no app can "do it all", it's necessary to move the "music" between systems without losing information on systems that don't fully understand every component in the "package" that is passed around. MusicXML is specifically designed to allow, for example, a guitar chord-building and diagraming app, to embed custom chords into the package, and apps that don't understand these diagrams can simply blindly copy these parts from input to output.

Other publication issues, such as laying out music with three or four parts on the same set of staffs, are often required, but doing so in BIAB is difficult. Even things like editing note durations and force are very tedious on BIAB, whereas other programs offer much easier methods for doing the same.

So for those who use BIAB/RB to create audio and MIDI, I don't see that MusicXML would be of great use. However, for those who have to also publish the music, lack of MusicXML makes BIAB/RB a non-mainstream application. Because BIAB/RB is so good at the unique things they do, it's a shame it's so difficult to use in conjunction with other music editing and publishing apps.

Just my thoughts - I don't mean to start a flame war ;-).