I agree with Mary (and other posters) that I use my windshirt a LOT. Against wind (over a Puffball vest perhaps), for that little bit of warmth first thing in the morning, around camp, to sleep for a little warmth and to help keep my expensive sleeping bag cleaner, against mosquitoes and tiny gnats trying to fly into my ears, etc. It breathes enough to not overhead while hiking uphill with a backpack on, unlike my rain shell.
If you are in California (Sierra), it's a great place for a windshirt. I use a Puffball vest, windshirt (Houdini), liner gloves, and beanie as my usual only layers (over shorts/t-shirt). If it's going to really rain, a rain jacket (or MLD poncho tarp) has to come along. Often you can be SURE it's not going to rain, and it's a perfect piece of gear that gets used constantly. And it can ALWAYS go along on a day hike, since it's 3 oz and compresses to nothing.