Have you actually driven in snow or mud? open diffs or not, 4wd or AWD will get you through stuff that 2wd with even much better snow tires isn't going to make it... Also in any 4wd or AWD car without too much electronic interference, you can drag the brakes lightly while using the throttle and it acts a bit like an LSD, but that's more of an off road maneuver. Now on public roads, in a fairly flat place, sure fwd and good snow tires does everything you need except driving when you really shouldn't be.
Add some bigger slopes, or unplowed roads, then AWD and 4wd do much better than 2wd just for staying mobile. Locking diffs isn't really an asset until you get off road or have much different grip on one side of the vehicle than the other.