A good pad below you is vital especially if you're on snow. The second you touch ground below you in a down bag, it's so cold you don't feel like your even in one.
I spent a hellish night sleeping on snow near the top of the Kaweah gap on a Thermarest pad because between the smooth floor of my tent, the smooth face of the Thermarest and my smooth down bag I would be off my pad with the slightest movement. To make it worse I was on a hill. I never used a Thermarest again after that trip.
