From what I’ve read in audio books, there are many times when our production music may be being played back in mono and we may not be aware of it, such as: radio gets folded down to mono at a certain distance from the transmitter; cable companies transmitting in mono but advertising as stereo to save $$; cheaper stereo televisions that aren’t really in stereo.
A good reason to check your mixes in mono! I tend to keep the stereo panning on the conservative side…