msycamore said:

The funny thing is that Dolby Stereo was designed to be Mono-compatible. So in a way it was unnecessary if you doesn't care about the actual audio content.

There was enough concern about that issue that Ben Burtt and company thought the dedicated mono mix was a good idea at the time.

We're probably lucky Fox didn't push for the use of Sound 360, (used on Damnation Alley the same year) as that system didn't catch on.

There's a nice article about the sound mixes here.

http://www.fromscripttodvd.com/star_wars_a_day_long_remembered.htm