Junebug - you brought up an interesting point. In my youth, I made several sheath style dresses and a back zip was all I needed. However, at the time, my waist was 13" smaller than my hips. When I made a few dresses that were 50's influenced, many had a side zip in addition to a back zip. For the 50's wasp waist the side zip is a bonus fitting detail. Less waist definition doesn't require one.
My body no longer has a need for a side zip.