Just saw this thread featured in the BPL e-newsletter and pulled it up since it’s a topic of interest. Didn’t know I had inadvertently initiated it, too.
I’ve already made my fleece v windshirt argument, but I’ll repeat a few points and offer some more context:
1. The aforementioned post was part of a long series in which I detailed 13 essential items of clothing that can be mixed-and-matched to create appropriate clothing systems for all 3-season conditions in North America and many other parts of the world. In this post, then, I’m really arguing that fleece is a more valuable “Core 13” item than a windshirt, and not necessarily that windshirts aren’t better in some situations.
2. A fleece may not block the wind like a windshirt, but it’s more insulative, and I believe that net warmth is about the same even in light or moderate winds. In calmer air, it’s definitely warmer, and less likely to trap moisture.
3. Fleece is about as warm as a windshirt, and it has an entirely other role, too: as a mid-layer between a hiking shirt and WP/B garment in cold or cold-and-wet conditions. As a mid-layer, a windshirt is relatively worthless.
4. If it’s so cold and/or windy that a fleece is not enough on its own, put on your WP/B shell. In those conditions, there will be more than enough airflow through the jacket (especially if your jacket has some venting features) to offset the limited breathability of a WP/B shell vs that of a windshirt.
5. As RJ said, there is minimal penalty for taking a windshirt. It’s really not necessary, but it will allow you to fine-tune your comfort better than a clothing system without this piece.
6. After a few uses, DWR-treated wind shirts are completely ineffective protection against precipitation, unless it’s so light than it’s evaporating as quickly as it’s falling, but in that case the experience would be no different than with a fleece.