Since I've been blessed enough to be able make 7 "out of season" trips to Utah and Colorado in the last 2 years, my OR Tantrum II windshirt has become one of my favorite clothing pieces for the desert in winter and western mountains in late fall.
Rare for me to use it in the Southeast, though. It doesn't fare as well in high humidity, plus I do too much offtrail hiking that it wouldn't survive.

It's always combined with a sun hoody that also doesn't see so much use where there's nearly 100% tree cover, but is my other favorite clothing piece out West.
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I do occasionally bring the windshirt in warmer weather if it's supposed to cool off a good bit at night.
Pros for me is that it's lighter and more versatile in that instance than a lightweight baselayer(though I often bring a long sleeve nylon button up in its stead), and much better than hiking in a rain jacket out West(where it's completely replaced the nylon shirt).
In addition to being far more comfortable to hike in, it helps me avoid putting wear and tear on my shells. I only break those out for very inclement weather, and/or big temperature drops where the windshirt isn't enough by itself.
Aside from a puffy for camp, a sun hoody, windshirt, and rain jacket make up the entirety of what I've been using up top for those trips.
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Whether a windshirt is worthwhile or not depends on when and where you plan to use it, IMO, and I probably wouldn't have much to say if my backpacking was limited to low elevation in the SE(which is the vast majority of it)...