One chap I knew that did a defensive drivers course half a lifetime ago was told never to have his window 1/2 way down. He was told it should be either fully up or fully down as in a side impact from the passenger side, the driver could loose the top half of ones head hitting a half open window (the glass top edge). That was before side air bags so it may not be so relevant these days.
Defensive driving courses have a lot to do with risk, accident and injury reduction/minimisation
Nothing to do with handling oversteer or understeer, rather, more on how to avoid getting into such situations in the first instance