Designing the rifle with the fine adjuster screw makes sense. Take a regged rifle; once you have set the reg pressure you have only the hammer spring as a form of adjustment; you wouldn't want to strip the gun to tweak the reg if you didn't have to.
Now take a knock-open unregged rifle, like the masses that are produced. You are looking for a flat top power curve. One of the easiest ways to get that is to set up the hammer spring to give you a little over 12 at the peak, then pull that back with the adjuster. If you need proof, just try setting one up purely on the hammer spring, and you'll get a curve like a banana.
The screw isn't there to provide the unscrupulous with an instant power hike. In most cases it gives only a small adjustment anyway, and someone looking to run a knock-open rifle at seriously high power needs to do a lot more than twiddle one screw.