Generally you gain 18 AB per season per lineup spot. So over the course of a whole season, a player batting one place out of their ideal lineup spot will make almost no impact. Add to that you are talking about a pitcher who will generally be pinch hit for after about 75% of the at bats and you are talking about maybe 13-14 AB over an entire season. Plus he generally did it when he had a pitcher with a higher BA than the position player. In general batting order makes little difference unless you are doing something crazy like batting your pitcher leadoff or your best hitter in the bottom of the lineup. Don't get me wrong, I think LaRussa is a pompus jerk, but I wouldn't fault him for batting a pitcher 8th instead of 9th.