A limited-slip differential is great at providing a degree of automatic correction for unanticipated loss of traction. As you point out, they work pretty well, but they don't guarantee traction in all conditions. As has been discussed on this thread, auto 4WD (if equipped) also helps to compensate for loss of traction, albeit in a very different way, and via a far more complex system. Most of the RWD and 4WD vehicles I've had over the years have been equipped with some form of an LSD, and my current vehicle has both an LSD and auto 4WD (and 4WS, which can make a low traction situation much more fun).
An electronic-locking differential enables the driver to prepare the vehicle for anticipated slippage, effectively pre-locking the axle vs. waiting for the LSD to kick in. This can provide a higher degree of control to the driver, especially if in a situation where sudden vehicle movement due to loss of traction would be undesirable. I've not owned a vehicle with an e-locker, but I look forward to testing it in a 'sticky situation' sometime soon.