I bring one in early spring and late fall because I tend to hike in a tee-shirt regardless of season, and add a light wool sweater when it drops into the 30s. If it's windy, I sometimes find that the sweater needs a little "boost" while I'm walking. By blocking the wind, but still being fairly breathable, the windbreaker works pretty well. If I don't need it walking, and it's windy in camp, the windbreaker is nice, too. It's not nearly as "stiff" as the WPB rain gear I always carry, so it's more comfortable when sitting around in camp. As it gets colder, I also carry down pants, booties, and hooded jacket for camp warmth and to bolster my down quilt. They're made of light polyester material, so the windbreaker again serves its anti-abrasion role if I happen to be wearing the jacket while hiking (almost never, but there is that occasional day when the mercury is reluctant to climb up the thermometer wall.)
For four ounces, it's just a handy thing to have around sometimes.