Wool kilt hose are necessary as the temperature approaches freezing. I generally wear them below 50°F, but not always. When I hike in the cold I'll wear knee-high gaiters to keep snow out, and accomplish the same as kilt hose. If I'm backpacking, I'll just wear the gaiters; if I'm only out on a day hike I might wear kilt hose beneath the gaiters.

I've worn a four-yard PV kilt in 0°F weather, and it's fine as long as you're moving around. Then again, in that temperature you'll be cold no matter what, so it doesn't matter much if you're wearing a lighter or heavier kilt. Get inside by a warm fire, you nut!

Andrew.