Originally Posted by aschick
Keep in mind that most if not all cars with part time AWD also have traction and stability control system, and that is mostly what keeps them on the road in bad weather. AWD itself will do nothing to help in a skid or to prevent too much power from being delivered to the wheels.
I agree traction/stability control can be a good thing, but I consider it a "nanny" of last resort, when the driver has either been caught by surprise or botched up big time. I do not think of them as being critical or required to keep an AWD car on the road. Though my current cars have these systems, many of my earlier AWD vehicles did not, and they drove fine in capable hands.
The better AWD systems will prevent too much power from going to certain wheels -- they can be proactive about shifting and transferring power. For example, one of Subaru's most basic systems will send power aft upon acceleration (when the vehicle squats and the front wheels unload a bit) and the SH-AWD system in my RDX transfers power to the outside rear wheel in corners, effectively overdriving it to reduce understeer and take power from the inside rear wheel (which unloads a bit). So I think the better proactive AWD systems can function intelligently by managing power before falling back on stability/traction control to handle the situation when something goes wrong.
I dislike traction control systems in many cases because they normally function by applying the brakes, and this greatly limits forward progress. This is used in lieu of a true limited slip diff, by applying brakes to spinning wheels and transferring power over to other wheels. The downside is that is eliminates half of the power being sent to that axle (Wikipedia has a good article on this for anyone who wants details). So exercise caution when you see AWD systems that are dependent on traction control -- they will suck in deep snow. They can actually reduce the power at each axle to 25% of the total power, and reduce total power going to the ground to 25% to 50% of available engine power.
My RDX has traction control (part of Honda's VSA) and I have to turn it off in deep snow or the vehicle will suck wind. I had the same experience on a rental Outback that had traction control. Luckily you can turn it off when you need to. I leave VSA on all the time, but turn it off before driving up a snowy hill or in any other deep snow situation where forward progress is important and the yaw/skid danger is low. The lousy thing about Honda's VSA is that you can't turn it off once it's kicking in, so you have to know and plan ahead of time and turn it off when it would get in the way.