The All Backs Motif is odd. For one thing, it casts a revealing light on we magician's claim (when it suits ourpurpose) that we use ordinary cards. That is our decks, which we prefer to borrow, consist of cards that havefaces and backs on both sides. Therefore, if we show a deck that consists of all backs and faces we are admittingthat such anomalies exist and the cards being used are probably gimmicked in some way. Why do this?

A better question may be - What motivated someone to create such an effect using this kind of deck? That "someone" was Dai Vernon and my guess is that he wanted to amuse other magicians and provide a plot whereby they could devise methods to magically transform misprinted decks into regular ones. Granted, this may not be THE reason but it's clear that the plot eventually inspired lots of magicians to figure out ways to do the trick with regular cards. In other words, at the outset, a regular deck is apparently shown to consist of all back and faces. Then the deck is shown to have reverted to a regular deck and all is well.