Ah, now I see where you are coming from Anth, we are talking to some extent at cross purposes. Crossing hands is a minor fault, it wouldn't cause you to fail a normal driving test, but could be counted against you on an advanced test (on the ADI part 2, you are only allowed to make 6 minor faults).
The reason that it is consider to be important for an advanced driver to passed the wheel through their hands, is that in order to do it smoothly every time requires the driver to take good observation all of the time. As passing the wheel through the hand is slightly slower that crossing hands, it requires the driver to anticipate the maneuver and adjust to the correct speed for the maneuver in good time. This need good all round observation and actually thinking through maneuver in advance, rather than mechanistically reacting to situation.
It is rather a subtle point I am trying to make and I not sure I have explained it fully. There is a reason for all these things, which is not always fully understood by those doing the teaching. Another thing which often misunderstood is just how fundamentally dangerous driving is, compared with other activities, just as riding a bicycle.
The thing that makes driving so dangerous is that it is possible to achieve high speeds with very little effort and that a lot of consideration is required to be safe. That is not to say that riding a bicycle is entirely safe (nothing is), it is just that to achieve speeds high enough to be at risk of serious injury does serious effort. Also most of the risk involved in riding a bicycle on the roads comes from the carelessness of those piloting 1.5 ton+ of metal with very little effort and sometimes concentration.