I wonder what Francis Chichester would have thought of the suggestion that the deviation card is redundant. Clearly nobody operates light aircraft for long distances over water any more. Knowing what the deviation is, is more to do with working out where you are, than being able to steer it. The average PPL could probably survive quite well without a compass, especially if they have a GPS on board.
At the end of the day though if we did not calculate the deviation we would not know how accurate the compass was! Having calculated it, it might as well be written down so that all can see how accurate the compass is.