I once did that at school and was told that it wasn't the done thing and should not do it again (I never did). Incidentally, on my occasion the batsman was given "not out" - I was told the ball was not live until it had left the bowler's hand, so to get an "out" I should have moved the ball from one hand to the other. Perhaps that rule has been changed or was a figment of the teacher's imagination.
I've read a bit more about this in the papers today, and the umpires were saying the man was legitimately out. However, etiquette still remains. In the Tour De France - a notoriously corrupt sport - riders don't seek to gain an advantage if an opponent has a mechanical problem. If they can abide by the etiquette that has been established over years, so should cricketers.
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