Much has been made of the fact that Jarrod Washburn is a southpaw and that, in a park that benefits left-handed hitters, it's not too bad of an idea to bring in a glut of left-handed pitchers to counteract said biased environment. This is true, and while the impact is frequently being overstated in an attempt to somehow justify the contract, it's definitely there, and it's real.
That said, lefties also do something else pretty well that's much easier to quantify: shut down the running game. Washburn didn't allow a single stolen base last year, and surrendered only three in 2004. For his career, just 51 runners have stolen a base off of Washburn in eight seasons, and while some of this undoubtedly has to do with the guy behind the plate, a lot of it is because of Washburn's left-handedness.
So, what kind of impact has shutting down the running game had on Washburn's ERA? It's actually pretty simple to figure out.
We have two things to investigate - (1) Washburn's ability to prevent stolen bases, and (2) Washburn's ability to pick off baserunners. Let's go in order and take a look at #1. Last season, there were 2566 successful stolen bases in 43230 total innings - an average of 0.06 steals per inning. Let's take a few liberties and assume that this has been the league average since the beginning of Washburn's career. If that were the case, then we'd predict that Washburn would have allowed 68 steals since breaking into the league - 17 more than he actually has. The calculated run value of a stolen base is 0.19, so that means that, for his career, Washburn has saved 17 * 0.19 = 3 (roughly) over the average. Hardly anything.
However, what if Washburn has improved his ability to control the running game in recent seasons, and that last year's zero steals allowed wasn't a fluke? Even if that's the case, it's not worth very much - Washburn saved just two runs above the average last year by keeping guys on first base. You could make the argument that it's a little more than that, because runners are less likely to take the extra base on a single or double against a lefty, but even then, you're probably only talking about one or two runs over the course of a season.
That brings us to point #2: pickoffs. These are worth considerably more than a prevented stolen base, because where blocking a steal simply prevents the one-base advance of a baserunner, a pickoff erases him entirely and adds an out to the scoreboard. Jarrod Washburn, for his career, has picked off 31 men, and at a run value of 0.43 runs a pop, that equals 31 * 0.43 = 13 (roughly) runs saved for his career. That's more than six times the impact of shutting down the running game. In an average year, Washburn will keep two or three runs off the board simply by virtue of making a quick throw over to first base and catching a runner off guard a few times.
In the end, what all this means is that Jarrod Washburn has saved anywhere from 15-25 runs over his career by being a left-handed pitcher who controls the running game - an ERA difference of 0.12-0.20 points. It's not that much, and it's probably not significantly different from the majority of other lefties in the league, but at least it's something; given a choice between a left-handed Jarrod Washburn or a right-handed equivalent, the left-handed version is going to be a few runs better because he doesn't let baserunners advance as frequently.
Of course, going forward I don't think this helps us very much, because Jarrod Washburn hardly allowed any stolen bases in 2004, either (3), and he still sucked. But it's nice to be able to put a number to something that a lot of people are talking about.