I agree with most of the comments, especially Dan's. Here is my take on it:
A windshirt is not necessary. I always carry the following while backpacking:
1) T-Shirt
2) Puffy Jacket (synthetic, but down would work as well)
3) Propore Rain Gear
This will provide protection from rain, cold, wind and bugs. In other words, I can survive and be reasonably comfortable with this gear. The puffy jacket weighs less than fleece. The Propore is very light, breathes really well, and is extremely waterproof. A tarp would be lighter, but not provide bug protection. So, as far as I know, this setup is the lightest setup that will provide protection from rain, cold, wind and bugs. There are flaws with this system though:
1) It is easy to be stuck "in between" hot and cold. It may be too hot for the puffy jacket, but too cold to go without it.
2) Propore is extremely fragile.
3) While Propore breathes well, it is not as breathable as most windshirts.
A windshirt fills in these gaps at a very low weight penalty (2 or 3 ounces in men's large). Putting a puffy jacket over a windshirt is only done for convenience.
Since I hike in the Pacific Northwest, I do a lot of day hiking. While day hiking, I carry the following:
1) T-Shirt
2) Fleece Jacket
3) Propore Rain Gear
Fleece breathes much better than a puffy jacket. It therefore has a much bigger comfort range. As a result, I don't need anything for "in between" temperatures. However, using Propore as my wind and bug protection is still just as flawed. I would hate to rip up my Propore jacket just to save a couple ounces, especially since I don't care as much about weight when I go for a day hike (which is why I carry fleece instead of a puffy jacket).


