Baseball has a ton of conventional wisdom that sabermetrics has proved wrong, but there are some things that I still take for granted. For example, I always believed the idea that having a fast runner on first will cause the pitcher to throw more fastballs. This made sense simply because it could pretty easily be verified by asking pitchers and catchers. This is supposed to provide some of the value that Juan Pierre and other scrappy guys bring to the table that stats heads supposedly miss.
However, recent research by John Dewan discovers that this actually isn't true. While pitchers do throw more fastballs with a faster player on first, the effect is negligible. Without a stolen base threat, pitchers throw a fastball 61% of the time. With a stolen base threat (10 or more steals), it goes up to 62.8%, with a major threat (30 or more), hitters see fastballs 63.2% of the time.
So, Juan Pierre causes hitters to see one extra fastball every 50 pitches. Does it matter much? I can't believe it does.