Let's talk about the fastball. The fastball is what made Walter Johnson the Big Train. The fastball is why Nolan Ryan is NOLAN RYAN!. The fastball made Colt Griffin millions as a first round pick. The fastball is why Daniel Cabrera still has a spot in a major league rotation and it's why Brian Bannister risks his career every start. The fastball makes scouts stand up and take notice and makes fans stand up and cheer. The fastball is the mythmaker of pitching. But how important is it really?
You'd probably say that a good fastball is nearly essential to pitching in the major leagues, and I'd be inclined to agree with you. Barring the handful (fingerful?) of knuckleballers and the occassional Jamie Moyer who float around the bigs, it's hard to find successful pitchers without some redeeming aspect to their fastball - be it speed or movement. But that's not very precise. And if you've read almost anything else I've written, you know I like to try to quantify things. Blame it on the Math degree that collects dust in my closet.
I wanted to measure the value of fastball speed - or how much was an additional mile per hour worth to a pitcher. So I took the full set of fastballs captured by Pitch F/X from 2005 (those pitches identified as a variety of fastball or over 88 mph). For every one of those pitches, I categorized it according to the speed of the pitch and calculated the linear weights run value to find the average run value for each speed category. The results are shown in this graph:
Of particular interest is the linear fit trend line which seems to do a fairly good job of matching the actual results - especially between 87 and 97 miles per hour. It's a little hard to see in the picture, so let me relay it here: y = .0002x - .0292. The key number is that .0002 which means that, in general, each additional mile per hour on the fastball adds .0002 runs per fastball thrown. That equates to .018 runs of ERA (technically, RA) for every mile per hour on the fastball.
Imagine two nearly identical starters. Say they each average 6 innings per start, and throw exactly 100 pitches in those starts. Of those 100 pitches, 60 are fastballs. The only difference between them is that Starter A throws an 85 mph fastball and Starter B average 95 mph. All else being equal, we'd expect Starter B to have an ERA .18 runs better than Starter A..
Obviously nothing every works out that easily in real-life, but there does appear to be a distinct advantage to throwing the ball faster. And while there is likely a selection bias here, I think it might actually dampen the real effect. We've all see pitchers who throw the ball harder get many more chances to succeed than a soft tosser. That would drive down the average value of the faster pitches, thereby lessening the observable effect of speed.
Another possible concern is that the linear weights values I used are slightly off. You'd expect the average run value across all measured pitches to sum to 0, but the sample I have (all pitches, not just fastballs) sums to about -2.75. That suggests that at least some of the weights are off (which makes sense since I didn't calculate them myself for my sample - I just took numbers off the Web). The discrepancy in run values could confuse the results of this study if they don't cancel out across the sample size (roughly 390,000 fastballs).
In some ways, I'm not really surprised there's an advantage to throwing a faster pitch. To think otherwise, you'd have to believe that nearly every decision maker in baseball history has been mistaken. That said, I have to admit I expected a minimal advantage to throwing faster, especially considering the apparent selection bias favoring those pitchers with lesser fastballs. This definitely explains why pitchers like Daniel Cabrera still have jobs, and pitchers like Colt Griffin or Matt Anderson can go so high in the draft.
 Starter B gains 10 mph * .0002 runs per mph or .002 runs for each fastball over Starter A. Over 60 pitches, that's a .12 run difference. Since each pitcher only lasted 6 innings, the .12 run difference would be .18 over 9 innings.