I can't help but think that some folk don't understand what a windshirt is for. Say you set out to climb a 3,000 ft mountain slope. The climb might take you 2 or 3 hours. When you are working hard, your body produces sweat to cool you down by evaporative heat loss.
This is fine in warm weather.
Climb that same slope in temps nearer freezing, with a steady 10-20mph wind. You will still be sweating, but the cold wind will chill you pretty quickly, maybe dangerously so.
Of course you can put on your rain jacket to cut out the wind. You will still be sweating, but the sweat has no place to go. Even the most breathable waterproof shell can't cope with the sweat produced by 2 or 3 hours of hard work. Your clothing then becomes saturated with sweat. At best this is uncomfortable, in winter, maybe dangerous.
So you put on your windshirt in place of the rainshell. Good windshirts are made out of something like Pertex Quantum or similar. I prefer as little DWR as possible. Your sweat is wicked into the windshirt from your lower layers. It then spreads out on the outside to evaporate off. The dense weave of the windshirt allows this to happen, but still cuts the wind.
If you never encounter cold temps or wind, or you never work hard enough to raise a sweat, then leave the windshirt behind.
For myself, it's the most used item of outdoor clothing i have, and i wouldn't hike without one.