When people say that they don't seem to understand that what they are talking about is raw forgings. There are only a dozen or so forges who make the raw forgings. I always get a kick when people list an upper by who's forging marks are on it. Doesn't mean much. They are forged to outer dimensions only. Who does the machine work and anodizing is what is important, not who forged the raw casting. Here is a picture of a raw forging of an upper. Lower is in the same degree of finishing from the forge. Manufacturers buy it from a forging company and do the machine work and anodizing. Aero makes many other brands out there. For a few bucks you can have them finished with your own roll mark on it. After you get the variance from the ATF.There are waaaay more than 4-5 companies that make uppers and lowers. Just off the top of my head I can name 12 quality manufacturers of upper receivers, and there are quite a bit more than that.
Even if 2 companies buy their lower receivers from the same manufacturer, this does not mean that both are equal. Different vendors will specify different levels of fit, finish, quality control, etc., from the manufacturer. These differences can greatly effect the overall quality of the final result, as well as how much they cost to make. Just because Company A and Company B both buy their receivers from Manufacturer X does not mean that they are the same receivers.
You are correct in that parts like the barrel are more important than the receivers.....within reason. Other primarily important parts include the BCG, gas system, and buffer system.