[RC-Monster Mike]

Well, there is no substitute for a smooth trigger finger, but a smooth trigger finger is also no substitute for a slipper. When you land a big jump on all 4 wheels, for example - the shock is transmitted through the entire drive train(even if your "smoothly" picking your nose with the trigger finger). The slipper will absorb this shock. When accelerating through a particularly rough area, there is also considerable shock. Once the shock force exceeds the slipper tension, some slippage will occur and absorb this shock - again, the trigger position has nothing to do with it.
Regarding traction control - well, you can't "feel" the track from the driver's stand, so your smooth trigger action, while helpful, is also somewhat reactive(and hopefully proactive on the 2nd lap). And unless your have motor skills equivilent to Rainman's math skills, you won't be exactly the same every time, either. The slipper setting will be consistant. On a slippery track, you can dial in slip and make it much easier to control traction. On a firm track, a stiffer setting will be transparent(won't know its there), but hard landings or other high shock loads will be absorbed by the slipper.

So yes - it can limit power transfer to the wheels like you mentioned - in fact that is the whole point. It limits "power" or "shock" both ways(from the truck to the track or from the track to the truck). :)