I've saved this little rant since I didn't want to rain on the fun, and Zimmerman clearly deserves the national attention that he's getting, but hitting streaks are stupid.
OK, they're not totally stupid. It's kind of a curiosity when a guy can get hits in many games in a row. But the way baseball fans and (mostly) media act like hit streaks are some grand achievements attributable to great skill and not (mostly) randomness is just weird.
Plus, if you are going to focus on streaks like this, why wouldn't you hype "on base" streaks? The guy reached base twice today and scored a run. Why is that regarded as a fruitless day? Joe DiMaggio reached base in 74 straight games, including his record hit-streak. The Baseball Reference "streak finder" goes back to 1954, and in that time Orlando Cabrera has the record with 63 games in 2006. But generally the record is neither recognized as significant nor conclusively known.
Aside from that, I have to say it was pretty dumb when Bruce Bochy intentionally walked Zimmerman in the 7th inning. Runners are on second and third, one out, and the Nationals are leading 2-0. At this moment, the Giants' win expectancy is 13.4%. Bochy walks Zimmerman to get to Josh Willingham and Elijah Dukes. By doing this, the Giants' win expectancy actually falls to 12.9%. The Nationals run expectancy for the inning started at 1.42 with one out and men on second and third. With the bases juiced, that number rose to 1.58.
There are situations where it would make sense to intentionally walk the bases loaded, but that requires a much more significant drop-off in the effectiveness of the batter you're walking and the batters you're choosing to face.
In The Book, Tom Tango, Andy Dolphin and Mitchel Lichtman calculate us the "break even points" for this difference between a hitter and the guys following him and when the IBB starts to be advantageous for the pitching team from a win expectancy point of view. Down 2 in the top of the seventh with runners on second and third, the ratio of the hitters' wOBA and the weighted average "teammate wOBA" of the three men following him must be at least 1.20.
Zimmerman was to be followed by Josh Willingham, Elijah Dukes, and Austin Kearns. Zimmerman is currently sitting on a .430 wOBA, which is extraordinarily high, almost equal to what Manny Ramirez did in 2008, and it's not reasonable to expect him to maintain that going forward. ZIPs just released their updated projections for every player in the league posted at Fangraphs, and they expect him to post a very good but not MVP-caliber .370 wOBA over the rest of the season, which seems reasonable (think Prince Fielder in 2008). Based on that, you'd need the hitters following him to average less than about a .308 wOBA (think Brian Schneider's '08). Willingham, Dukes, and Kearns are a lot better than that. It's not even close.
OK, but you say, Zimmerman's hot. If he's a .430 wOBA hitter so far this season, we should treat him as such. This is kind of silly, playing into a kind of gambler's fallacy that just because the ball has landed on red three in a row, it's sure to land on red again. We know that Zimmerman isn't Manny Ramirez, regardless of what the small sample size says.
But whatever, let's consider this. To make the IBB make sense for a .430 wOBA hitter, you'd need to the weighted average of the three guys following him to be less than .357. The ZIPs "rest of the season" projections for the Willingham/Dukes/Kearns trio gives you a teammate wOBA of exactly .357. So it's basically a wash, and you ruined everyone's fun.
Now, let's do the same calculation using their actual 2009 wOBAs. The Willingham/Dukes/Kearns trio this season combines for a weighted teammate wOBA of .346. The drop-off is mainly because Willingham and Dukes are both off to somewhat slow starts; Hammer in particular is 15 points below is career wOBA. Still that gives us a Zimm-to-teammate wOBA ratio of 1.24 (and remember, we needed a ratio of 1.20 to justify the walk).
So is that it? Have we cracked the Bochy code and proved that he did the right thing after all? Well, only if you completely ignore sample size, assume that Zimm will continue to vastly out-hit his historic performance levels, and Willingham and Dukes will significantly underperform theirs.
And I guess that's what we've learned here: Bruce Bochy (as well as Manny Acta, Dave Jageler, and Zimmerman himself, since they all said the move was a no-brainer) doesn't understand sample size.