You are absolutely right, weight is a major downer in airplanes of any kind and the effects of it are pretty easy to determine. My lead sleds fly pretty badly but are good for golf clubs and can't be broken. Landing speeds go up and maneuverability goes down.

But if you have two airplanes of similar light weights, low wing loadings, high power loadings etc. then you might also see the differences between a carefully selected airfoil and a WAG airfoil. Being careful about the selection just makes sure a good design isn't hurt by a bad choice in airfoils and is something that is easily remedied in the design process. After all it's no harder to build and cover a good airfoil that a bad one.

Plus you have the mental good feeling knowing that you have done everything the best you can, and that will make the airplane fly better. Airplanes we don't really like usually don't get our best piloting efforts.