In the current implementation, reversing lights in the Model S do the job to warn traffic from behind. They do almost nothing to light up the area behind the car to let the driver see where s/he is going. This is particularly true for the European version where there is only one reversing light (the corresponding space on the other side is occupied by the mandatory rear fog light).
This is a shame b/c it renders the wonderful HD backup camera almost unusable in dark situations.
The problem is exacerbated by another issue that has been filed separately: Disable mirror darkening in reverse
The new Mercedes S class seems to have found a novel (and surprisingly obvious) solution for this problem: There the reversing lights are implemented as a band of LEDs that are integrated into the rear diffuser, i.e., low and center.