Good questions
How is it that card counting is still crucial to your game even though the casinos use several decks and reshuffle before even getting near the end of the set of cards.
First of all, there still exists single and double deck games. Ignoring the 6/5 abominations, single deck games can be found in downtown Las Vegas, Reno, Laughlin, Mesquite, and few maybe a few venues outside Nevada. Double decks are common most places. In any case, multiple deck games are beatable by counting.
The most common misconception is that a counter memorizes the entire deck. That would be difficult in single deck, let alone 6 decks. In reality all we do is keep track of the difference between high and low cards. If the remaining cards are rich in high cards, that is advantageous to the player. In these situations we bet more.
The difference between counting single and multiple decks is that the removal of a single card is more pronounce in single deck. The player has an sizable advantage with the removal a one 5 in single deck. In 6 decks, you would need to remove six 5's to get the same effect. The result is that high counts are less frequent. To compensate, a counter playing 6 decks will use a larger bet spread or avoid playing when the count is to his disadvantage.
One advantage for a counter playing 6 decks over single or double decks is that they usually receive less scrutiny from the casino. This is particularly important for those betting large quantities.
Another misconception is that counters can predict the cards coming out and therefore know when to hit/stand. While it is true that a counter may vary his strategy based on the count, he has no way of knowing which of the remaining cards is going to come out next.
The second part of your question deals with deck/shoe penetration. Deeper penetration helps a counter because it increases the frequency of higher positive counts. While it is true that penetration is not as deep as in days past, most games have adequate penetration to be beatable. A casino does not want to shuffle too often, otherwise too much time is wasted that would be better spent taking money from players.
All in all, I guess the answer to your question is that blackjack is still beatable by counting, but not as good as it used to be. Outside of exploiting weaknesses in casino procedures, card counting (including techniques that incorporate counting, such as shuffle tracking) remains as the best probable method of beating blackjack.
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