I remember the time when we moved from mono to stereo in the 60s - the discovery that humans had two ears was one of the revelations of the era . But several tracks played on SOTS don't work in stereo - the mixes don't combine in mono and things get lost (like, sometimes, the vocal). When we mixed my tracks we listened to the final mixes four times - big stereo, small stereo, big mono (for clubs) and small mono (for radio) before OKing them.