There's also another way at looking at posi vs open diff - an open differential 4x4 in a very poor traction environment will only guarantee that one wheel on each axle will turn - a posi/LSD will guarantee you that 3 wheels will turn. That third wheel turning more often then not is the difference between getting there on time or getting there late or not at all.
I also think that if you find yourself unable to keep your vehicle on the road with your LSD that you'd have the same problem with an open differential. You may feel the rear end "dart" one way or the other occasionally on slick surfaces, but you will have to have the "Luck of the Irish" to lose control of your vehicle due mainly to the LSD as this probably only happens once in 3 lifetimes so the odds are on your side. Lockers will cause a few more "darts" but again, you won't lose control unless you're really into the carburetor and have lost control of your senses. I'll take an LSD over an open differential any day of the week and I'll take selectable lockers over the LSD any day of the week, but that's me.
