Casaleenie,
I ran an open rear end in my Cobra for years. Usually, in normal driving, it just doesn't matter. However, if you're trying to set a fast time on an autocross track and you power out of a turn, the inside rear wheel will spin, keeping you from accelerating. As Karl pointed out, that's not all bad. On the road, that spinning inside wheel on a slippery curve will keep you from going around backwards (spinning out). It's the original traction control.
For the racer types, this takes all the fun out of four-wheel-drifts and opposite lock steering. Personally, I like hanging the rear end out and when I rebuilt the rearend I had Hooper's set the preload tight. Now, I am constantly alert for loss of traction in the rear and ready to quickly snap the wheel over to catch it.
Paul