Interesting thread! A few more comments about the windshirt.
-I find a windshirt useful when I anticipate snow but I need very little insulation- for example, snowshoeing in deep snow when enery level is high, and I could probably do fine with a merino LS shirt by itself, but need to stay comfortable in the likely event that a pine bough decides to dump its snow load on my head.
-In high activity, cold temperatures, no snow or precipitation, where one is going to sweat a bit even in a LS merino shirt, I've had moisture as either condensation or frost gathering in places on a merino LS shirt when that is my only layer. In these conditions, would a windshirt simply provide an additional barrier to this moisture, or might it help me deal more effectively with it by creating a buffer area of warm air?
-For nights in camp, would a windshirt be worth its weight, as an alternative to a less beathable shell, over a puffy synthetic/down insulation layer, particularly in situations where the dewpoint might be inside the insulation? The windshirt might move the dewpoint closer to the exterior of the insulation layer, or even put the condensation point on the inside of the windshirt itself- and deal with the resulting condensation better than a gore-tex or eVent shell.