This depends on how something was recorded and how you are listening in mono.On that point Rob thinking back to when I was a kid with a mono record player. When you put a stereo record on it, you almost lost part of the sound in the odd place. Does this not happen converting a track back from stereo to mono?
Sometimes old mono devices were just the left channel, so anything that was panned right disappeared. There was a low wattage independent radio station that was set up like this, so every time they played early Van Halen, there was only guitar reverb and no guitar. That would not have happened if it was both channels collapsed to mono.
Also, back in the early 70's, people were experimenting with stereo by putting drums on one side and vocals in the other and goofy stuff like that. If you were only listening to one channel, you might not be able to tell what song it was.
If you have a well balanced stereo mix, and the 'stereo-ness' doesn't come from copying and phase (polarity) inverting an instrument, you should be able to collapse both channels to mono without completely losing anything.