Aye, that's the thing: base layer, wicking intermediate layer, insulation, wind-proofing, and rain protection. All my layers (excepting perhaps rain gear) can be worn comfortably on the skin, so I can use any combination that suits the conditions, sleeping, and laundering.
I just switched off from a vest to a long sleeve pullover insulation in the realization that I can eliminate a spare long sleeve base layer and get better cold protection with about the same weight. It extends my sleeping bag range too. It's all about maximizing every facet.
Your description is good-- when the poo hits the ventilation system, all that is in your pack is spare underwear and you look like the Michelin Man. You can wear two pairs of socks too :)
You know, what I would like to see is an insulation payer that doesn't duplicate my wind protection. Fleece is an example, but compresses poorly and is heavy. What I have in mind is a garment that has a lining, high loft synthetic fill, and some sort of bonded gauzy mesh outer layer to be worn with a wind shirt.