Re:- scratching your back - you've lost me a little on that one. I think I understand what you are saying but if I understand you correctly the same logic says that a sandal is better for hill walking than a boot as the sandal enables you to scratch your foot if you have an itch?
Yes, it is. That's one of the reasons I much prefer hillwalking in sandals... Sandals weigh less, breathe better (make that much, much, much, much better), feet in them dry quicker, socks are not needed. Downsides... not much use for scree running, foot jams, winter weather, but no shortage of conditions outside of those.
It may be slightly heretical not to use boots, but that doesn't mean it's a bad idea!
As for breathability of a back covering, if you've got, say 5% direct coverage of mesh against the back that's 5% less breathability than if it wasn't there, would be my guess. After all, it's been suggested that changing from standard to narrow seam tape on a waterproof makes a tangible difference, and even a fine mesh is more of a barrier than none at all. But if the tension can't be maintained without anchoring it at the top, that's a pretty good reason to anchor it at the top! But if you could keep the tension just from the side mountings, then less would be more as far as contact comfort is concerned, at least where you're not taking a load.
Pete.