Indeed, back in the day most systems were mono, and so it was assumed that most people would be listening to it (be it a movie, LP, the radio, whatever) in mono. Therefore, sometimes a lot more care was put into the mono mix. i.e., in many cases, the stereo mixes were novelties, whereas the mono was the "serious" mix that the mass success of the product depended on. I can remember when you'd buy an LP, you had the choice between the mono and the stereo version. The mono wasn't just a sum of the stereo, but more often were entirely different mixes. Most engineers back then knew what they were doing, and knew that a summed-stereo mono would suffer from all kinds of cancellation artifacts, so they'd make separate mixes. And for the reason mentioned above, more often than not the mono version was the better one!

Great examples were some of the mid-60s bands like the Beatles, Donovan, etc. The stereo mixes were often obviously rushed and lackluster, but the mono mixes kicked ass. Many people feel the same way about the Star Wars original mono mix. I don't feel strongly myself either way, but I do think it is VERY important to preserve mono mixes when they are available - often they represent the producers' best effort at the time.