I agree with you on this. For the life of me I can't see how a slightly, or even horribly inconsistent first reg affects the shot if the second reg is consistent. An inconsistent first reg is no different than having no first reg and the bottle pressure, which will be lower after every shot, is what's arriving at the second reg.
For instance the first reg varies between 150-160, so that is what arrives at the second reg. Now remove that first reg. Bottle filled to 250, as you shoot is goes down to 150 over the course of your shooting day. How is a 100 bar swing not affecting the shot but a 10 bar is??????
I think people just fixate and consistency. If it's not there it drives some crazy, whether that particular thing affects the goals or not.