Designated Hitter

Currently, MLB rule 6.10 (the designated hitter) allows a team to replace the pitcher in the batting order with a replacement batter. I wonder: why isn't this rule more generalized to allow replacement of any one defensive player? I ask because there are quite a few pitchers who are better batters than the weakest everyday player on some teams.

For example, there have been many cases in history where a National League manager chose to bat his pitcher 8th, ahead of a particularly weak-hitting position player. The most recent example is Dontrelle Willis, whom Jack McKeon batted twice ahead of Robert Andino, a very light-hitting shortstop. You can see more info about pitcher hitting 8th on my post over at Baseball-Reference.com here.

There have been quite a few pitchers with great batting seasons. Wes Obermueller hit .385 for the 2004 Brewers. Mike Hampton has hit for both great average and great power in recent years, before his injury troubles. Since 2000, Brian Bohanon, Jason Marquis, Jason Jennings, and Carlos Zambrano have all had seasons above .300.

Forget simply batting these guys 8th. Why not hit them 9th, but use the designated hitter to replace some weak-hitting middle infielder or the catcher? OK, so the National League doesn't have the DH rule. But, NL teams could use such an expanded DH rule when they hit in AL parks during interleague games or the World Series, and there are probably quite a few pitchers in the AL right now who could hit better than some lowly guy, such as Julio Lugo of the Red Sox.