I can see how the player would have a big advantage in a double deck BJ game if he could turn all of the tens one way and the dealers didn't rotate the cards when shuffling and used an shuffle machine. With a hand deal you may be able to see the end of the next card to be dealt.

In a six deck game you could not only know the dealer down card but also maybe the next card out of the shoe. Some of the decks I saw had the ends different but both sides of the cards were the same. That would make it hard to see what the next card would be in a shoe.