Nick Adams is obviously correct. Under an ideal scenario of the perfect high speed turn, the LSD offers no advantage.
But in many other situations, including non ideal ones like powersliding, saving an oversteer moment and basically have more rear-steering control, it provides a great advantage over the open diff; and that's why almost ALL racecars and track oriented sportscars have one.
For example, I don't think I would have saved this one having had my open diff Elise instead of my LSD Exige. Once that rear wheel is up in the air (A bit too much rebound) there's no power or push to keep the car from spinning into the Armco.
(save as)
http://homepage.mac.com/c.gomez/.Mov...lOversteer.mov