In addition to the wind resistance, I also find a windshirt adds a fair bit of warmth to my layering system in colder weather. I often wear one over a merino top, and under an down jacket. I think the windshirt helps trap warm air close to my skin, preventing convective heat loss from air circulation inside my clothing.
Plus, if you spend much time hiking UP exposed ridges in moderate weather, windshirts are essential. Raingear is too hot, and no-protection is too cool. Buy one, and I bet it becomes one of your most-used, most-carried pieces of gear.
I think the ID Pertex jacket is sweet, or the Patagonia Houdini. Or you could make you're own with a thruhiker.com kit for only about $40.00!