Just wanted to contribute my own thoughts here.
Like others, I won't make the argument that windshirts are a matter of necessity, or an emergency "essential." But, they are a joy to use if you happen to know how, and be in the right conditions.
Being small and warm-natured, I both warm up fast and cool off pretty fast. This means that when I go to the New Mexican Rockies in the summer and I'm gaining a bunch of elevation and then stopping to rest (in the intermittent wind and [maybe] drizzle), I'm putting my body through a wide gamut of changing, um, thermodynamics. Right? First I'm hiking hard uphill below a ridgeline, shielded from the wind, panting, and sweating . . . and then I'm resting on the ridgeline, exposed to the wind, and cooling rapidly. This cycle repeats.
Now, I definitely could get through all this by repeatedly changing out layers, as others have mentioned. I could take off my pack, strip/add my layer, pack/don it, and put my pack back on, over and over again.
Or, I could just wear a windshirt.
A nice breathable windshirt is the only layer I know of that can take me through all these conditions without a lot of discomfort. It protects enough, it breathes just enough. A windshirt just seem to fit that range of exertion and weather perfectly. Not to mention, most of them are fairly cheap, really light, and reasonably durable. Make a good choice like a Houdini, or Pertex, or (my favorite) a Montbell stretch, and you've got shower protection too.
Better explanation, maybe?