In the world of broadcast it is normal to have multiple different audio configurations.
e.g. a live game will be covered 5.1 surround, with a 2.0 folddown. This produces 2 x 5.1 and 2 x 2.0 for full mix & international
a show to be edited might have multiple 5.1 sources (full mix, audience, fx), 2.0 (music, VT) and 1.0 (isolated mics)
a drama might well have a sound recorder with a 2.0 guide mix and multiple 2.0 and 1.0 sources (fx / isolated mics) and include MS material - this may well be correctly configured in FCX if the iXML data is correct, I haven't tried that yet, but the data is often incorrect.
In tight turnaround (or low budget), the audio is finished in the NLE - no audio post.
There's much to like about FCX - I give time to give it a serious look at every iteration. And it is getting closer (some stuff would be super useful, but it falls down in the audio department every time for me).
~ My FCPX Babbling blog ~
~"It is a poor craftsman who blames his tools."~
~I still need to play Track Tetris sometimes. An old game that you can never win~
~"The function you just attempted is not yet implemented"~
Have you messed around when sharing Master File? For audio choose Multitrack Quicktime Movie and you can easily send roles out as anything you want. Just for fun I just created a 16 track QT file with random roles as 5.1, stereo, mono in totally random order to create a mess and it all came through exactly as I shared it showing a total of 16 tracks with 2 5.1's i stereo and 2 mono. The tracks have the role name so you could easily create a Project with lanes for specific roles in whatever order you needed for deliveralbes. Maybe I'm crazy but seems pretty easy and cool!
It was at a Vegas premiere that I resolved to become an avid FCPX user.
I agree that the roles designation on export to multiple tracks works well - and is reflected in the audio configuration if exporting Quicktimes. Sadly I don't think MXF has the ability to designate anything other than as a mono track - and thus you cannot recreate the audio configuration when importing the resultant MXF back into FCP. The choice is 16 mono or 8 stereo on a 16 track file.