I am not sure if it is a matter of bad craftsmenship or not as wood is not perfectly straight. there are bound to be some warping or crowing of the wood.
Plywood helps in covering the studs which would show the most imperfections, by averaging out the imperfections over a span of the piece of plywood. The insulation I think might do a little bit more.
Each consecutive layer spreads the issues even further and over a longer distance.
Like joint compoud on new drywall. Each coat is applied over a larger and larger surface area. The high spot is still there, but if it is compared to aspot right next to it, you will not be able to tell,but if you were able to take a laser to it, you will still see that the high spot is still there, and actually got a bit higer then original...
Not real sure about that analogy, but it is out there now...
Your point about wher ethe suppor tis located is important though. if the frame is on the edge, the support should not transfer it to the bottom glass, again that is what I was told.. And in one line of thought, it makes sense..
I wish I used it, even if iwas just for the edges. I dont see how it could hurt as long as the force remains ont he edge, and like you said, the insulation does not give that much in compression
Thanks,
Bill