Depending on who you ask, pinch hitters are either the unsung heroes of competitive teams or affirmative action for fatties that couldn't cut it in the AL. As a distinctly "National League" type role for a bench player, pinch hitters are often deployed in key strategic moments within a game, where a single at-bat could potentially turn the tide and help change a game's outcome. These high-pressure situations lend themselves to particular types of players who embody the requisite mental fortitude and desirable skillsets needed to excel in this role. With the trend towards generalism and versatility in position players, the late-inning hitting specialist is rapidly disappearing.
There are several different archetypes for pinch hitters. Many Padres fans will think of sluggers like Matt Stairs, who belted a record 23 pinch hit home runs in his career. Others will remember the more subdued style of Lenny Harris, who's 804 career pinch hits will probably never be surpassed. Others still will think of Kevin Towers' several experiments with the pinch hitter position - a strategy that usually consisted of former quasi-stars like Tony Clark or Cliff Floyd thrust into the role. These familiar tropes appear to be slowly eroding in the new data-driven era of the sport, where rosters are constructed on overall value and production rather than situational excellence.
Sabermetric-minded teams tend to favor players with skillsets that are quantifiable with volumes of data - the more data there is to support a player's skillset, the more resistant that claim is to sample-size variance and confounding variables. Pinch-hitting data is small-sample by nature. Furthermore, since pinch hitters are often employed in "big moments" in a game's trajectory, a hitter's performance has extra tendency to activate our own cognitive biases. These phenomenon together make it difficult to evaluate pinch hitters effectively, however we can still isolate the skillsets that made the game's most effective pinch hitters so successful:
Patience
On the whole, relievers tend to be more wild than starting pitchers. High-leverage relievers in particular tend to rely on overwhelming velocity or movement on their bread-and-butter pitches. Many of the best pinch hitters to suit up for the Friars were adept at working the count and laying off of bad pitches. Matt Stairs, John Vanderwal, Dave Hansen, and Mark Sweeney all had career walk rates in excess of 12%. To put that number in perspective, the 2013 Padres managed to draw a free pass in only 7.6% of their plate appearances.
The trade-off of this patience is that the deep counts tend to result in an elevated number of strikeouts. Strike outs can take the proverbial wind out of the sails of a budding rally, but a when a pinch hitter forces a deep count from a fireballing reliever hopped up on adrenaline, he wears them out and gives his teammates a chance to scout and time the pitcher's fastball.
Fastball Hitting Ability
Patience is somewhat useless if it doesn't end in a walk or get you a fastball to hit. Unfortunately Pitch F/X stats were not kept until 2002, but between that time and the end of his career Matt Stairs did as much damage on fastballs as the entire 2013 Texas Rangers roster. This adds to the list of baseball's timeless axioms: #475: don't talk about a no-hitter while it's in progress. #476: when you enter the game, the ball will find you. #477: don't throw Matt Stairs a two-strike fastball.
Pinch hitters tend to face late-inning relievers, who tend to rely much more heavily on plus-fastballs. When Edgar Gonzalez was miscast as a major league baseball player pinch hitter, the lesser Gonzalez often fell behind in the count early on fastballs and rarely recovered. Players who would otherwise make decent role players could still struggle in the pinch hitter role.
Makeup
Call it clutch, call it composure, call it swag (#lmao #yolo #kony2012). Most people walking this earth don't have the nerves to handle this kind of high-pressure job - especially knowing that even the best are going to fail most of the time. Lenny Harris, widely regarded as the greatest pinch hitter of all time, was a career .269 hitter. On the surface there is nothing special about that batting average. But if you put a .269 hitter in Lenny Harris' shoes, he won't hit .269.
Just like how most relievers are not cut out to be closers, most hitters are not cut out to enter night after night with the game on the line, against the other team's best relievers. To some extent, all hitters are groomed to deal with more failure than success - even Mike Trout makes an out 60% of the time - however, failure in high-leverage situations carries a kind of gravitas that strikes at players in a more fundamental way. A pinch hitter who strikes out with the winning run on base in the 9th might be called again in a similar situation the next night. Hitters groomed for this role need to develop a special type of resiliency to failure, and a thick skin that allows them to work unfazed in big moments.
Scouting the Current Roster
Tommy Medica and Kyle Blanks are two bat-first prospects on the peripheries of the current roster. Both are basically useless in the field - limited to first base or short stints in the outfield corners. Both have exciting bats and have impressive minor league track records coupled with some major league success. As a result of this, there has been considerable fan pressure to provide them with at-bats on a team that often is perceived as offensively-challenged.
Thrusting either into the pinch-hitter role at this point is tempting but misguided. Younger players almost never break into the league as bench bats, and when they do their development curves are seriously disrupted (see: Belt, Brandon). Inexperienced players just don't have the job-specific work ethic, confidence, or inertia to immediately take on one of the game's most difficult jobs without room for error or practice. Baseball is a game of repetition and execution. For young players to develop into contributors, the first concern in their development is accumulating the maximum number of repetitions against quality competition in the shortest period of time. Giving a young guy spot starts and a pinch hitting appearance every third day isn't going to allow him to develop the habits necessary to become a great player.
Thanks to roster-flux, there is no cemented bench player in the two ostensible pinch hitter roles. Theoretically, the jobs were supposed to belong to Chris Denorfia and Seth Smith - experienced players with exploitable platoon splits, and excellent fastball-hitting ability. With the absence of Carlos Quentin and Cameron Maybin, both of these players have seen their roles pushed closer to the everyday level, which promises to expose some of the weaknesses in their games. If Smith and Denorfia are allowed to return to their situational roles, they can provide excellent value to the team from a sabermetric standpoint and be used to great effect from a situational one.
Bud Black's favorite ninja, Alexi Amarista, has also been employed off the bench in an attempt to serve as a kind of spark plug for the offense. He is far from the typical pinch hitter archetype, and is the owner of some unimpressive traditional stats. However, he is Bud Black's Swiss Army knife off the bench since he can play almost anywhere on the diamond and has uncommonly good bat control. Despite being a relatively weak hitter, he strikes out about half as often as the rest of the team - making him a great candidate to hit behind runners or move them into scoring position. In an ideal world, I don't think Amarista is Budbot's first option off the bench, but he certainly has his uses
Finally, although Yasmani Grandal and Yonder Alonso are both pegged as young regulars at this point in their careers, they both possess many of the requisite offensive skills to one day make good pinch hitters. Alonso fits into the bat control mold with the added perk of decent walk numbers and a good sense for fastballs. Fans are still waiting to see if the power aspect of his game will ever develop, but his advanced approach is unlike most hitters with his experience level. Grandal is an on-base machine who also sports above average bat control. He has flashed power-average potential already, but his main skill is his patience at the plate. Since entering the league he is in the top-5 in bb%, ahead of Jose Bautista and Adam Dunn. While they're games are not perfect, Grandal and Alonso are already showcasing a type of acumen that is normally the province of far more experience players. While I personally believe they will pan out as everyday players, they could one day prove worthy of a roster-spot based on their situationally valuable tools.
Outlook
It certainly appears that teams are less likely to devote entire roster spots to late-era Mark Kotsay types who project to see just 150 or so plate appearances a year. Thanks to renewed emphasis on defensive value, the trend has been towards more generalized (and typically more athletically gifted) bench players who can fill in for injured starters for longer periods of time rather than the bat-only fossils of yesteryear. These players can log more meaningful playing time and generate a more descriptive body of work with their statistical profiles. However, the value of timely hitting will always be high, even if it is more difficult to scout and evaluate. As front offices continue to try to race eachother to the next great market inefficiency, there may be a renewed interest in players with the skillset required for late-inning heroics. Finding and developing these players will be challenging, but the payoff is always tremendous.