The best source for free, non-traditional statistics just got a little better, as FanGraphs has added catchers' Stolen Base Runs Saved to their calculations of Wins Above Replacement (WAR). Honcho Dave Appelman:
For the most part, all catchers will remain about the same in value, especially on an individual season basis. But certain ones, like Yadier Molina, ends up with an extra 3.6 wins over the past 6 years. On the down side, Jason Varitek probably sees the biggest decrease in value, with -2.2 wins being attributed to his catcher defense since 2003.
As you probably know, 3.6 wins over six years isn't a huge number of wins. But it's both measurable and worth measuring, and I'm glad it's been added.
Of course, there is so much more we could do. Just off the top of my head, wouldn't it be wonderful to have a systematic account of how catchers handle pitches in the dirt? Sure, we can count passed balls. That's moderately interesting, except they're quite rare. What's less rare are pitches in the dirt, some of them caught or knocked down, some skipping past the catcher to the backstop. To this point, nobody's figured a way to incorporate those pitches -- 90-some percent of which might be blocked, theoretically -- into a catcher's defensive rating. The ones that get blocked (with no advance by the runner) are essentially ignored; the ones that escape the catcher (with a runner advancing) are assigned to the pitcher as a wild pitch.
But you could give credit to catchers for blocking pitches in the dirt, with the amount of credit depending on the difficulty of the play. A pitch that bounces three inches before reaching the glove is generally more difficult to corral than a pitch bouncing a foot in front of the plate. Also, a passed ball thrown by Tim Wakefield shouldn't be counted the same as a passed ball thrown by Jamie Moyer. With the Pitchf/x data, there are (or will be) ways of figuring all this stuff and more.
Evaluating catchers will always be tough. But it's not impossible, and will only become more possible in the next few years.