The insulation comes from having low thermal conductivity. Fleece has really low thermal conductivity. So its cold on the outside and warm an the inside, the fleece creates a barrier between the two, otherwise the natural need to reach equilibrium would cause you to loose heat. The wind stopper prevents the heat you disepate into the fleece from being blown away... It would work on the outside better, but then it'd be noisey and rip, trust me the windstopper makes a difference. fleece is warm up to 8-10 mph winds, then its like wearing a tissue because the fabric is not dense, which is what gives it the warmth..

That being said, I use alot of fleece, I buy all camo outer wear fleece garments and wear them, when the wind picks up I put on my lightweight weight rain gear over top... to put the wind barrier on top.