On slippery roads (snow, ice, rain, etc), having any sort of posi or locker in the rear can increase the chance of the rear end sliding around when the vehicle is turning a corner or such. It's especially bad for pickups since we don't have much weight over the rear tires.
If the rear differential is open, then when one tire starts spinning, the other one will act as a rudder, helping to maintain a straight line.
Having said that, it's possible to spin out in an open rig, and it's possible to drive just fine in a posi/locked rig. You just need to adjust your driving style to accommodate the particular quirks of the rig.