I always thought that the main value of a lightweight DWR windshirt was in misty conditions, such as encountered in the OR/WA Cascades. However, I keep using it more and more: to keep off biting flies during rest stops, to keep off the wind when it's too cool and windy to hike without a wind jacket but far too warm to be comfortable while moving in a WPB jacket, to put on during rest stops if it's breezy but not cold, even in the sleeping bag at night so my inevitably dirty shirt doesn't transfer its dirt to the sleeping bag. My Montbell windshirt weighs 2.4 oz. and is well worth its weight, IMHO.
As for the conditions for which I originally bought the windshirt: If it's warm and drizzling, I just get wet. since I'll get just as wet from sweat inside my rain jacket or even the windshirt. My body heat dries out my synthetic shirt and pants in 10-15 minutes once the precipitation stops. If it's cool and drizzly, I wear the wind shirt--if it's more than a mist, I get a bit wet, but the extra layer keeps me from getting chilled. If it's cold enough that I don't sweat inside while hiking, I wear the rain jacket.