Use of the designated hitter (DH) in the MLB

Question

With the Houston Astros moving to the American League starting in 2013, each league (National and American) will have an even amount of teams (15). As a result, the debate regarding designated hitters (DH) is heating up. As it currently stands, the AL uses designated hitters while the NL does not.

Why are designated hitters used in the AL, but not in the NL? Are there any specific reasons why the AL adopted the rule and/or the NL did not?

One would think that this rule would be used MLB-wide to keep things consistent...rather than having to play by the respective rules of the home team's league during interleague play, when a player gets traded from one team in one league to another team in the other league, etc. However, the MLB claims that the DH has become "a role more than a player."

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History₍₁₎

Pitchers have historcally had low batting averages. In 1973, American League owners voted 8-4 in favor of using a designated hitter to replace a pitcher's at-bat. Since, the AL has generally had a higher batting average than the NL. The DH was first used in the World Series in 1976 and for even-numbered years until 1985, where the DH would only be used when games were held at an AL team's stadium (when interleague play (NL vs. AL) began in 1997, the DH would be used in the same fashion). These days, the DH position is used for players to rest and take a partial day-off rather than a full-time position. Also, the DH position has diminished the use of the double switch in the AL, but is still rather prevalent in the NL.

Answer 1

The AL was swayed by Charlie Finley's (the Oakland A's owner) argument for having a DH, believing it to make for a more interesting game, that it would increase scores, and revenue (from here, here, and here - this third reference is the best, in my opinion):

By the early 1970s, Charlie Finley, the colorful owner of the Oakland A’s, had become the designated hitter rule’s most outspoken advocate, arguing that a pinch-hitter to replace the pitcher [...] would add the extra offensive punch that baseball needed to draw more fans.

The NL wanted to stick with tradition, and did not have the support for the addition of a DH role.

At the time, the NL was beating the AL in scoring and attendance (reference, reference). That may be why the NL was happy to stick with tradition, but this is only a guess on my part.

Another good analysis is by D. Buehler (J.D. University of Washington) and S. Calandrillo (J.D. Harvard Law): Law, Economics, and the Designated Hitter Rule, published in the Boston University Law Review.

They summarize the history as follows (taken from their chapter titles):

• Dominant pitching and decreased attendance create a favorable climate for changing the rules
• Baseball adopts various rule changes to increase offense, including the DH rule in the AL
• The DH rule increases offense and attendance, but the NL refuses to go along

Reasons for why the NL refused to go along include:

• Baseball fans disapproved.
• Ted Williams was against the rule, predicting it to be "the forerunner of other things. More specialists. More substitutions."
• NL President Chub Feeney thought that the change to baseball strategy would take away entertaining moments of the game
• There were personal player objections to the change. For example, Jim Palmer (pitcher for the Orioles), felt that he wouldn't be as big a part of the game sitting in the dugout. Hank Aaron thought that as a DH, one wouldn't have the opportunity to redeem oneself on the defensive side of the ball if he struck out.

Later, in 1984, the MLB Commissioner polled fans and learned that AL fans favored the DH rule, but NL fans opposed it.

The analysis by Buehler and Calandrillo goes further, including a discussion of moral hazard, and how the DH rule changes the cost-benefit of pitching inside, increasing the number of hit batters in the AL. However, this isn't part of the argument made by the NL for opposing the DH rule.

They conclude:

American League fans enjoy the increased offense that the designated hitter rule provides. National League fans enjoy pitching duels and the chance to see the manager struggle with the decision whether to pinch-hit for the pitcher in close games.