I think it's a very good point. My current plane (project) is finally in the ballpark. I can actually fly a complete pattern on it, though it's still a bit too exciting. The problem is, the plane weighs about 68-69oz. Now, it's close to 770 square inches, so I thought it would be able to carry the weight OK. The engine, an OS46VF, seems to have plenty of power, but the plane is a dog. It's a dog, not because of the weight (I've had heavier planes that flew much better), but because of the design. It's largely a Bill Werwage wing with a fairly thin airfoil. The thing just doesn't have the carrying capacity for that sort of weight. If I can figure out a way to get 2 or 3 oz off the tail, then I can take the 5oz of lead out of the nose and get the thing down to around 62-63oz and it will probably fly OK. But as it is, it's a pig. I could probably up the pitch on the prop and increase from the 5.3 second laps I've flying to the 4.8-4.9 range and alleviate some of the problems, but certain not "fix" them.
If, on the other hand, I had used my favored airfoil (more like Ted's airfoils), I suspect that, while lighter would be better, it would fly just fine where it is.
I feel like it's a combination of design numbers, weight and available power. Lighter is better generally, but not always. I think there is a weight envelope for a particular design and power combination. Stay within the envelope and you have a good flying plane. Overweight creates certain problems while underweight creates a different set of issues.