With interleague play about to start in earnest, I've decided to take a look into whether AL teams unfairly benefit from interleague play over NL teams. Why would that be the case? Some argue that although both the AL and NL has an equal number of home games in interleague play, the AL has a bigger advantage because of the DH rule.
When games are played in AL parks, both teams get to use the DH. But the AL teams usually have a strong hitter playing that position while the NL teams will usually take their best bench player, who is generally not in the same class. But when the teams play in a NL park, the AL team will usually play its DH in the field and that will level the playing field, since now both teams can only use its eight best hitters and the top eight hitters are usually fairly equal across the leagues.
So for example, when the Red Sox play the Mets in Fenway Park they have the advantage of playing David Ortiz, while the Mets will probably DH Julio Franco. That's a huge difference. But when they play in Shea Stadium, Ortiz plays the field, like Carlos Delgado, and everything is fair.
I believe the AL does have an advantage, but it's not as big as people make it out to be. The above argument is overstated because the comparison is not between Franco and Ortiz but between Franco and Kevin Youkilis (the Sox's first basemen). Youkilis is the guy who is playing Ortiz's natural position because Ortiz can DH (let's call him Youkilis Player Y). If the Red Sox played in the NL, they would play Ortiz at first and probably not have Youkilis on the roster. So the advantage the Sox have in interleague play is Youkilis's bat vs. Franco's, not Ortiz vs. Franco.
Another advantage is defense. Players DH because they are a worse defensive player than whoever is playing their natural position (Player Y). If that wasn't true, they would be playing the field and Player Y would be DHing. Ortiz plays DH because he's a worse fielder than Youkilis. Same thing with Giambi and Andy Phillips. However, this advantage is illusory because the NL team will also play their better defensive player in the field and DH the other player. So the Mets might play Franco at first and DH Carlos Delgado in Fenway, which will negate the fielding advantage. So the AL's advantage is basically the difference between the best bench player on the NL team and Player Y (in our example between Franco and Youkilis).
That gap is not that large. Moreover, the NL does have advantages in their ballparks:
1) Assuming team payrolls are basically even across leagues (excluding the Yankees who are an outlier), the AL team is at a disadvantage. Let's say both the AL and NL team has 100 dollars to spend. If the AL team has a DH they will be forced to spend x number of dollars on the player who is playing the DH's natural position (Player Y). The NL team can spend the x dollars on other positions. So while the AL team will have a good hitter on the bench in NL parks (because Player Y won't start), the NL bench and bullpen (or other positions) will be better because they can spread that money out on the other positions. And when there's no DH, it's more important to have a deep bench and bullpen, because if it wasn't, NL teams would just spend x dollars on one slugger to keep on the bench. So the NL team will have the advantage here.
2) The DH in the AL will often play DH most of the season (and when he doesn't he's probably not that much better than the NL team's best bench player). But when he is forced to play the field in NL ballparks, he is at a disadvantage because he is not used to playing the field. If Ortiz played 162 games at first base, he'd be a better first basemen than he is now (when he only plays around 20 games a year in the field). If Ortiz played in the NL, he'd be a better fielder. Practice makes a player better. So even if the NL team has a bad player playing first, he'll likely be better than Ortiz simply because he plays more. And that's a disadvantage for the AL team.
3) On the same note, pitchers will probably be better hitters and bunters in the NL because they get more practice. While the disparity won't be great (pitchers suck in both leagues), this is still a small advantage for the NL teams.
Are these disadvantages outweighed by the AL's ability to have a better hitter in AL parks? That's an empirical question, but I'm too lazy to run the numbers right now. But I think the advantage is minimal at best.
By the way, interleague play has been pretty even. As of today the NL has 1,126 wins while the AL has 1,116 wins.