Here is my take on it: In most cases it is a luxury. So is a pillow (use a rock-stone like Bob Marley). But it is a very light luxury item that comes in handy for a lot of uses:
1) As mentioned, as a compliment to a poncho. This is a very light system that can provide protection for wind, rain and bugs. Unless you are carrying breathable Cuben, this is probably the lightest system that provides really good protection for all three.
2) It compliments fleece for day hikes. For a lot of my day hikes, I bring a fleece jacket and a windshirt. I leave the rain jacket at home if the forecast calls for 0% chance of rain. Worse case scenario I high tail it down the trail to my car (I always bring a rain jacket when backpacking).
2B) The same combination works great for skiing. I sweat a lot going uphill, so the fleece is usually just fine. But if it is really coming down (especially as slush) then I'll throw on the windshirt. When I go downhill, I'll probably have both a windshirt and down jacket on.
3) It adds a middle layer when combined with a puffy jacket and rain jacket. A very lightweight puffy jacket (like the Cocoon Hoody) is much warmer than a fleece jacket of a similar weight. Because it is much warmer, you can get by with a lighter sleeping bag. The drawback is that a puffy jacket is often too warm. In other words, you are too cold for just a base layer, and too warm if you put on your puffy jacket. The windshirt solves this problem by providing a nice middle warmth.
In all these cases it provides a much more comfortable layer than a rain jacket, which means that when I use it for bug or wind protection, I am a lot more comfortable. As mentioned, it is way more durable than Propore, which means I am a lot less likely to damage my gear.