Switch-hitting is a relatively new thing, at least in terms of the long history of baseball. Among the 50 switch-hitters with the most career plate appearances since 1901, only eight played before World War II. Eighteen of those began their careers in 1970 or later. Mickey Mantle was a phenomenon -- the first switch-hitter with power -- but Maury Wills was the switch-hitter who proved to be an important influence.
Wills was a fast guy who slapped the ball around and used his speed. Wills helped bring the stolen base back into the game in the 1960s after it had basically disappeared for several decades, and with the advent of artificial turf, an entire generation of speed players were made to switch-hit and told to pound ground balls through the infield and, hopefully, a few line drives in the gap.
There are unanswered questions among some evaluators about whether Hamilton will be a good enough hitter from the left side, in his continued development as a switch-hitter, and whether he’ll be consistent enough to be a productive everyday player in the big leagues.
As a right-handed batter in Triple-A, Hamilton hit .250/.317/.331; from the left side, he hit .269/.285/.369. While the splits appear similar, the big difference is in his strikeout-to-walk ratio: It was 64/34 from the right side versus 38/4 from the left side. He clearly has a better approach from his natural right side, albeit not with any increase in power.
But that's the trouble with switch-hitters: Once you've done it, it's hard to give it up, even if the numbers suggest it's a bad idea. Remember U.L. Washington? He was the shortstop on the Royals' 1980 World Series team, although you probably remember him for playing with a toothpick in his mouth (yes, the '80s were awesome). In 1980, his OPS was 160 points higher from the right side. In 1982, he hit .323/.367/.581 from the right side, .266/.323/.330 from the left. What if he had remained a right-handed batter? Eventually he stopped hitting from either side.
Look at Shane Victorino. He's always been a better hitter from the right side. A recent injury has forced him to bat right-handed against right-handed pitchers. Guess what? He's been great, hitting .310/.403/.500 in those matchups (in a small sample size of 67 PAs). It's entirely possible, however, that Victorino would be a better just hitting from the right side.
Another case right now is Matt Wieters. Lofty expectations were placed on Wieters after he hit .355 with power in his first season in the minors in 2008, becoming the top prospect in baseball. While he's reached 20 home runs for the third straight season, he's also hitting .232 with a .287 OBP. His Wins Above Replacement have declined from 5.1 in 2011 to 3.6 to 0.6 -- you're just not that valuable of an offensive player with an OBP below .300.
The low batting average has been fueled by a .241 average on balls in play, so it's possible he's been hitting into some bad luck, but Wieters hasn't hit for average from the left side for a couple seasons now. He has some power -- 11 home runs -- but his overall line is .215/275/.370, well below the .270/.318/.526 line from the right side. He had a similar split last year: .715 OPS versus .908. For his career, his OPS from the left side is .707 versus .823 from the right side (and trending worse after hitting well from the left side his first two seasons).
Wieters' switch-hitting has been a hot topic in Baltimore the past couple of years. He's obviously unlikely to stop -- Mariano Duncan did it early in his career and J.T. Snow eventually stopped hitting from the right and stuck to hitting left-handed -- a decision that paid off, as his production against left-handed pitching improved -- but for the most part, once a switch-hitter, always a switch-hitter.
Hamilton is a much different type of hitter from Wieters or Snow, of course, and hitting left-handed certainly helps him beat out more infield singles. It remains to be seen if that will make a better hitter, however.