It's an interesting point about the speed of thrust reduction. I was always taught (as per FCTM) to reduce slowly to reach idle on touchdown. I have flown with many guys that flare and then whack it against the stops.
Having watched the difference in these techniques I can say that there isn't all that much difference that I personally have witnessed, because as mentioned above it does take a small amount of time for the N1 to reduce. A lot of the guys I have watched that slowly close the thrust (like myself) actually close the thrust at approximately the same rate as the n1 reduces if sharply closing the thrust levers.