I will jump back in here since I had a specific concept of layering in mind. The concept is base layer, insulating layer(s) and weather-proof outer layer. What I am saying is that to achieve a certain "R" value and weatherproof, you can have multiple layers or one. Lightweight insulation such as down or a synthetic down substitute has to be contained between an inner and outer layer of nylon, which adds weight for the same "R" value. Ditto for that extra zipper, pocket, etc. One jacket, with sufficient down, with wind-proof outer material, probably weighs less than any "layers" of fleece or fleece, wool, down combinations. I have not seen any data that rigorously tests this hypothesis. Since jacket next to skin is not very comfortable, a wicking inner layer probably is needed regardless.

The major reason for layering, thus, is the flexibility of different combinations for different temperature/ weather conditions. But how many conditions do you really need to cover? Depends on when, where and style of backpacking.

When I hiked the Lost Coast (8 days - rain every day, temperatures only varying 5 degrees day to night), I started out with my normal layering and ended up hiking in my birthday suit under rain clothes, keeping only one set of light wool long johns and shirt dry to sleep in. Never used my "layers" because once they got wet they stayed wet, and ended up as heavy "boat anchors" inside my pack.

Second example, on a 3-week ski tour in very cold (high temps 0-degree F), very dry conditions, we had wool long johns and balaclava that we never took off. We skied in a wind suit. Once in camp changed into a hefty parka/with hood and down pants, ski mitts, and felt mucklucks. Quickly cooked dinner and went to bed. That was the "layering". It was so cold that parkas did not even need to be water resistant. In this case the "weather proof outer" layer was not needed. It was actually too cold to snow.

So, over the years I have been trending to less layering.