It would also be useful to allow users to choose to output stereo channels for the master, but just one mono channel for headphones. Audio interface outputs are always at a premium, this would allow people to choose to free one of them up. For me at least, mono or stereo makes no difference in headphones - I'd gladly sacrifice stereo in the headphones for one more audio out that I could use for something else. I'd want to keep the master output in stereo though.
It would also be useful to allow users to choose to output stereo channels for the master, but just one mono channel for headphones. Audio interface outputs are always at a premium, this would allow people to choose to free one of them up. For me at least, mono or stereo makes no difference in headphones - I'd gladly sacrifice stereo in the headphones for one more audio out that I could use for something else. I'd want to keep the master output in stereo though.