Makes some sense, but I'm still not sure if it makes sense to me. If I were racing over 72 hours where every ounce counts, I would take a windshirt over a fleece. However, the only ounces I count are the ounces on a longer trip.
For instance, when I did my 2012 bike tour around the northeast, I brought a 50º synthetic sleeping bag and a fleece jacket, along with a synthetic baselayer and then bike shorts and a light tech tee. When it was warm, I wore the bare minimum. When it cooled, I threw on the baselayer. When it was really cold and I had to sleep, like on our coldest night at one summit where we graced the upper 30's, I put on all my clothes, my fleece, and my raincoat and crawled into my bag. Comfortable!
A windshirt is great for standing on the summit, but it does nothing to help for actual low temperatures- just wind. Therefore, it can't be used in a sleep system like a fleece or a down puffy can. Am I correct in this line of thinking?
It's great for saving ounces for active hiking, but if you're carrying a sleep system anyways it becomes redundant.
Relevant extra facts: I took a synthetic bag and a fleece over a down bag and a puffy jacket for that trip because of expected wet conditions, and both were soaked at some point so I was glad for it.