AdvanceTrac helps, but it's a reactive system. It sees that something isn't right and then tries to correct it (albeit very quickly). Depending on the problem it may or may not be able to correct accordingly.
Things like dedicated snow tires, tire chains, and 4x4 (when activated in advance) are proactive. It's only after they fail that a reactive system, like AdvanceTrac, would take over.
Essentially, AdvanceTrac is no substitute for tire chains. Even with 4wd it's a good idea to use dedicated snow tires or tire chains when the conditions call for it. They'll do more for your safety than both 4wd and AdvanceTrac ever will.
But without them, it won't steer or stop on the 20's!
Please don't tell me it's all about the driver. I watched the Navigator slide down a snow covered driveway one day. It was parked and had been still for several minutes before it let loose. I was standing on the porch and nobody was inside. Scary! All-seasons aren't if you get more than the occassional dusting.