"How much do aerodynamics truly benefit the average bent rider?"
How do you define "benefit"?
If there is some kind of prize for finishing a ride a few minutes sooner, I guess that would be easy to define. If you're talking about satisfaction from being able to ride a little bit faster, I'd think that would vary from individual to individual.
Personally, within a range I don't care how fast I ride and my range includes a speed at which aerodynamics don't help very much. In my case I'd have to respond "little to none."