Sac Bunt: The 1-Run Gambit
The general defense of the sac bunt is that it is useful in certain situations to help get one run. This defense acknowledges that by employing the sac bunt, the team is essentially saying "We're giving up the chance to score more than one run this inning in exchange for more certainty about getting one run." As Barry Svrugla said in his chat, it is a bit of a "surrender." But does it really give you more certainty? To help determine this, below is the probability of scoring one run at each of the base/out situations. It is based on the 2004 MLB Situation Run Probabilities from Basebal Prospectus (subscription required).
The green boxes show that bunting with a man on first and no outs decreases your chance of scoring him by 2.5% (from 42.7% to 40.2%). The other situations also show a decrease, except for runner on 2nd, no outs, which increases your chance to score, but only slightly, by 1.8%
In essence, it is a wash to bunt to get one run -- it doesn't improve your chances to score that run, and actually decreases them slightly.
So, a team should never bunt, right? Wrong. If you think the batter is more likely than the average player to strike out, pop up or make an out that doesn't advance the runner, you should bunt. If you think the batter is more likely than average to hit into a double play, you should bunt. But, in both of these situations, you are essentially minimizing or avoiding a loss, not increasing your chance to score the run. For example, with a runner on first and no outs, you want to avoid a strikeout, because that reduces the chance of scoring by 15% (42.7 to 27.3). If you think the batter is going to strikeout (e.g. a pitcher with no clue), bunting makes sense to keep your expected run probability from dropping too much.
Now, with this in mind, you should read the next post on what Bill James currently thinks about the sac bunt strategy.