Chris, the reason I like the helmet light is it lets me look around cornors on downhills, and also by looking right & left as I approach an intersection I can make the light move in a wider arc than my handlebar light, hopefully alerting traffic on the cross streets that I'm there. I use both bar lights and the helmet for that reason.

I find the disc brakes really very helpful in the winter. When you ride through brown snow that has been whipped by auto tires it sticks to your rims too much to trust your rim brakes to be working when you need them. I can feel a lot more confident on the downhills in 3 to 6 inches of snow. Previous years I rode with rim brakes all winter, but after the last 2 winters with discs on the rear, never again in snow. If you go through mud the same would apply.