I have used my windshirt in temps which have ranged from 0F to 75F, in spots with no wind to 50 mph blows, and sunny skies to sleet and rain. Most of the time my hood is not up. I prefer a hat to maximize ventilation and visibility. But when the wind really starts blowing, or when it is getting cold (<40F when wearing just a base layer), the extra protects around the neck can make a big difference.
For example, earlier this season I was tromping through some blowing snow and 28F temps with my daughter wearing a Montbell Light Shell as an intergrated base layer, Montbell Thermawrap Vest, PolarBuff around the neck, a OR FallLine Hat, and my trusty Dragonfly Pullover Windshirt. [Oh, Cloudveil Rodeo Pants and Sahalie 2oz tights] I was chilled with the hood down and hiking, but when I stopped and I put the hood up over the PolarBuff and hat I was very comfortable. I think the windshirt helps more than you would think because not only does it block the wind but it helps contain pockets on air partially trapped by the insulating layer.