Interesting debate.
I appreciate that there's already an MC setting to force 1 channel audio to be output on both stereo channels. But I think MC's mixing capabilities are distinct from handling the very special case of 1 channel recordings, especially when they are intermixed with 2 channel recordings in a library -- or even a library with 1, 2, and 4 channel recordings.
I don't see this as an audiophile matter, since there's no natural behavior when feeding 1 channel of audio into a 2 channel system. Where should the 1 channel go? Why should it ever go to just 1 of the stereo channels? Which channel? If there exists some special case where this is desired ("1 channel source to left channel only") that might be an MC option, not the default.
This is contrasted with "mono" that is 2 channels with identical audio, as on CDs that present pure mono source recordings. This situation already plays "normally" because the producer puts the same audio on both stereo tracks.
Another example: Musical instruments, amps, etc. I have lots of this (play in a band) and the gear that provides stereo output if two cables are connected automatically switches to mono (mixes the source channels) if only one cable is connected. All the jacks are labeled this way.
Jumping past raw cabling situations, listeners normally hear 1-channel sources as 2-channel mono (and 2-channel stereo ends up as mixed 1-channel mono, such as was designed into listening to a stereo FM or TV station on a non-stereo receiver). Of course, if MC was intended to be a MIXER of multiple audio inputs, such as for recording/producing, it would need to support complex channel routing... but it's not.
At the digital file level, 1 channel audio is 1 channel for a reason, which I believe is simply to not waste file space, rather than to create a left-channel-only listening experience. Anyone know of a case of the opposite?
I think MC's default behavior should force 1 channel audio in to 2 channel audio out, separately from the mixing setting. This would mimic what CD producers do with mono source, and as was intentionally designed into playing a mono vinyl record with a stereo cartridge, and happens when playing a mono (aka "full track") audio tape.
(Certainly there are output devices -- cards, drivers, amps -- that don't touch the sound. That's a good thing, but that's not MC's only or even primary role.)