You really think I don't know how to sum two signals to become one Mono? You really think that I don't know that using an ordinary "Y" Cable backwards is wrong? You really think that I don't know how to configure my DJ software or switch my mixer to get Mono out? You really think that I don't know what a "Send" on a Live Mixer does?
Go ask many of those who just grab one side and think what they have is mono. Go ask those who take a stereo pair of speakers and put a side in one room and the other side in another room and think that is fine. I see it happening all the time here and out there.
...and why do you care?