The example of degrading a wall assembly due to air leakage was just one we all have seen. But in some threads I have heard it expressed that air leakage renders fiberglass insulation worthless. Where the air leaks into a wall cavity and back out (as in my attic example) that heat is lost. But where the ait leaks in and then continues into the home, no added loss at all.
You are correct it would either be too small of a number or at least too difficult to quantify, but in terms of keeping our thinking correct, it may not be as bad as we (note I'm included) thought. It's like the old, "you can't compress fiberglass" expression because it reduces its r-value. It reduces its thickness, therefore less insulation, but, up to a point, compression increases the "R" per inch.
Am I defending fiberglass??