Probably has little difference whether or not one shifts into neutral a traffic lights, but there are a couple of factors to consider:

(1) If one regularly does this, you are probably shifting the mechanism and valve/solenoid about 10-40 times more than the engineers anticipated, e.g., in 50k miles you will have "exercised" the mechanism about as much as the design engineers expected to happen in 500k - 2 million miles of use. Odds are the valve/mechanism is over-designed enough this doesn't make a difference, but I probably wouldn't do it.

(2) I seem to recall that in a motorcycle safety class I took that they recommended against being in neutral at stop lights for safety reasons -- if something happens where you wish to move quickly, you must shift into gear first.

Personal decision, but since the transmission is designed with the expectation that people will not shift into neutral, and since BMW auto transmissions are a known weak spot of BMWs, I leave it in drive.