Interesting. I live in the upper midwest and remember my first home I lived in (parents built it new in 1980) had returns in every room. My current home is primarily forced air and there are returns in every room with multiples in the larger gathering spaces. Was built in the late 90s and I think building code may require a specific number of returns per square foot.
Could be regional -- I would bet that in most cases houses in the upper midwest have to deal with much colder temperatures in the winter than houses in the PNW. Maybe that influences the HVAC design.
But I wouldn't be the least bit surprised to find that there isn't really a specific reason, it's just "how we do things in this area." A surprising amount of building choices seem to be made that way.
