[SAump]

The directive to take signs while on the rubber prevents the pitcher from taking them from anywhere else in the IF. So the pitcher must have the ball in hand, stand on the rubber and take his signs. Otherwise, how can the umpire direct the batter to stand in the batter's box and begin play?

This would also prevent both the pitcher and the batter from camping out between each pitch. The umpire would certainly be within his right to warn and eject here. Now if the rule had any teeth, a "ball" would be added to the count if it warranted a delay of game penalty.