The league-average batting-average-on-balls-in-play (BABIP) has been on a downward trajectory for the last half decade, something that has implications for the values of different at-bat outcomes. BABIP is down precipitously this season (.289), though it's hard to tell to what extent that's due to all the shifting defenses (better data on where balls are hit have enabled this), or is simply just an April cold-weather/small sample anomaly.
Here's the league-average BABIP by year since 2007:
| Year | League BABIP |
|---|---|
| 2012|STAR| | .289 |
| 2011 | .295 |
| 2010 | .297 |
| 2009 | .299 |
| 2008 | .300 |
| 2007 | .303 |
|STAR| Through April 30
As BABIP declines, the value of a strikeout declines, and the value of a walk increases. We can see this by looking at extreme cases. Let's say league average BABIP were zero, i.e., every ball in the field of play became an out. In that case, strikeouts would be nearly worthless as putting a ball in play would have the exact same result. The only value a strikeout would have is that it couldn't become a home run. But since homers are relatively rare, strikeout rate would have almost no predictive value in a zero-BABIP environment. But walk rate would be huge. A walk, a SB, and two sacrifice hits would be the most reliable way to score runs in such a scenario. Pitchers with the best walk rates would almost certainly be the best.
Conversely, in a 1.000 BABIP environment, every ball in play is a hit. So strikeouts would be the only way to get an out. In that world, K rate would be the only thing worth looking at, and walks would be nearly irrelevant, no worse than pitching to contact.
So you can see as BABIP tends toward 1.000, K-rate is more important. And as BABIP tends toward zero, walk rate is more important. As BABIP has dropped for each of the last five seasons (and ball-in-play data is enabling more efficient defensive positioning), walk rate is growing in importance while K-rate is declining.
That doesn't mean we want to ignore K-rate as we're talking about a shift from .303 to .289, not .189. But it does shift the emphasis ever so slightly to pitchers with better command rather than more dominance.