Something like this would definitely would be a wonderful feature for composition purposes. (I just did a YouTube search for "Cubase Arranger Track" to see it in action.)
I'm in the middle of a song right now - specfically, trying to move a chorus to earlier in the form - and finding it really difficult. (I'm
not a Sonar Ninja, though!!!!)
Granted, Sonar is a DAW - and perhaps such things should be worked out ahead of time - but it seems like there's an opportunity in this space. After a quick Google search, most "songwriting" programs appear to be lyric-oriented. Perhaps there's something else out there where you can move sections around and find that what you thought was a great bridge is actually a better chorus - or a song just flows better with a different form.