Just because the book says to split a certain pair against a certain dealer up-card, that doesn't necessarily mean it's a winning play. All it means is that splitting in that particular situation is your best alternative. Take the infamous case of having this pair for example:
How do you like it? Lots of players disagree with the book on this one and swear they lose money by splitting here. Well the truth is -- splitting 8's against a 10 does lose money! But splitting this pair over time loses less money than hitting or standing with your 16. So the thing to do is just grit your teeth, split 'em and save money as time goes by.
It's the same story with lots of other correct pair splits, but not all of them. Roughly one-third of the pairs that the book says to split lose money, and are merely the least costly way out of a bad hand. The other two-thirds are outright moneymakers.
So is there any need to know which is which? It would help your game if you did know, because there's are ways to handle pairs more effectively than just by splitting them when you're supposed to. Here's an example.
How many times have you seen somebody at your table with the following hand, and he doesn't know what to do?
Lots of players feel squeamish about breaking up an 18. But the truth is, splitting is an outright moneymaker here, because 9 against a dealer's 8 is a 55 percent shot to win! So the thing you as a bystander should do, is encourage him to split -- and offer to go partners on it! You'll both be picking up a 10 percent edge on the deal -- and it's not even your hand!
You can also occasionally save some money on your own losing pairs splits, but this play may be a bit ethically sensitive. Suppose you have the following hand:
The book says to split and you should, because that works out better than hitting or standing with 12 against a 4. Nevertheless, this is one of those splits that will win less than 50 percent of the time. Splitting is merely the smallest loser of the three options. But -- there may be a fourth option!
You can comment to your tablemates that you think this is a bad hand, and are happy to go partners on a split with anybody who likes it. Since it's a correct split by the book, they might like the hand better than you do and go halves with you. Naturally, they'll only take it if they think your hand has an advantage and you already know better, so therein lies the moral issue. Correct basic strategy splits that are losers are listed below:
2/2 against 2, 3 or 7
3/3 against 2, 3 or 7
6/6 against 2, 3 or 4
7/7 against 2, 3 or 7
8/8 vs. 8, 9, 10 or A
9/9 against 9
When you're dealt any of these pairs, going halves with one of your tablemates on a split will cut your expected loss on the hand in half.