From a new interview with Jed Hoyer:
Why don’t more trades take place between teams from the same division, assuming it’s not a salary dump? If all things are equal, I would think strengthening one’s team while weakening a divisional rival is a market unto itself.
JDH – The challenge with such deals is determining whether or not you are weakening your opponent. Or are you patching a hole they have and making them better—not just this year but going forward as well? I do agree that limiting your pool of available players by four teams is not good business practice. I think that trading within the division is fine -— provided, as I alluded to before -- that you aren’t giving your opponent the missing link or links.
Well, that's the rub, isn't it? You don't want to make him better and he doesn't want to make you better, and those twin impulses make it hard to get anything done.
One recent exception: Last November, the White Sox traded Chris Getz and Josh Fields to the Royals for Mark Teahen ... but I think that happened because 1) the Royals were trying to save money, and 2) the White Sox don't see the Royals as a legitimate threat in the division. I believe it's been 13 years since the Red Sox and Yankees made a deal; in 1997, the Sox traded catcher Mike Stanley to the Yankees (and the Sox got Tony Armas, who they later sent to the Expos in the deal for Pedro Martinez).
Now, 13 years isn't an especially long time between trades, but you get the impression the current regimes wouldn't even consider a trade. Which strikes me as bizarre. No, you don't want to give your opponent the missing link ... But what if your missing link is being filled, too? What if you think you're filling your missing link better than you he's filling his. And what if -- I know this is a crazy hypothetical, but please bear with me -- what if there's something called the "wild card" and you both, without those pesky missing links, can wind up in the playoffs?
On paper, there's little reason for the Padres to avoid dealing with the Dodgers, the Red Sox to avoid dealing with the Yankees, etc. But, as we know so well, there's a lot of stuff that doesn't happen on paper.