I use numerous apps that allow me to select a particular audio unit, virtual or otherwise. I'm not trying to select multiple audio outs — just one, and I would rather not have to keep changing my default audio unit just to run Stage Traxx through my desired audio unit.
I can get by as-is, but it's a source of irritation having to keep changing my default audio out in the system prefs just to make sure that Stage Traxx uses my desired audio unit.
I find it hard to believe that from a programmatic standpoint that you can't access all of the available audio units and not just the default setting. Literally all the other audio-based apps I use have equal access to all my virtual and physical audio devices — and allows me to choose which one I wish to use.
This is EXTREMELY handy as a performer both from a live standpoint and live streaming online where my audio routing is a bit more involved.