Baseball’s winter has taught us two things. One is that you do, in fact, get fleas when you lie down with dogs. The other is that the only thing teams value more than an ace is a second one.

Each of the three biggest trades of the winter saw a team that already had an ace shipping off a decent chunk of its farm system for another. Two months ago, the Arizona Diamondbacks sent Oakland six prospects, four of them very highly rated, for Dan Haren, who posted a 3.07 ERA in 222.2 innings last year and complements former Cy Young award winner Brandon Webb. Two weeks ago, the Seattle Mariners surrendered to Baltimore 22-year-old Adam Jones, one of the more valuable young outfielders in the game, and several prospects, all in exchange for Erik Bedard, who led the league in strikeouts per inning last year; he and resident phenom Felix Hernandez should prove a devastating pair. And, of course, the Mets decimated their farm system for the rights to pay Johan Santana $150 million and pencil him ahead of Pedro Martinez in the rotation.

Trades of this sort have become increasingly uncommon in recent years, because teams have come to realize how rarely they make sense under baseball’s economic system. An average player costs about $7 million a year on the free agent market; by fielding a player such as Jones, who can do the same for the league minimum salary, a team essentially saves that much money. If even one or two of the prospects Oakland picked up pans out, they’ll perform at a level worth tens of millions more than they’re paid up to the time they’re eligible for free agency, and getting more wins than you pay for is, from one perspective, the point of baseball.

In some cases, this doesn’t much matter. No one questions the Mets for trading for Santana, and it made similarly good sense for Arizona to send off their slew of prospects. Haren, for one thing, will also be worth tens of millions more than he’s paid over the next few years, as he’s signed to an almost punitive deal that will pay him $21.75 million over the next three years, less than half what he’d earn as a free agent. This is also a team that came a few games away from the World Series last year while fielding a number of young players that have the same or greater potential than outfield prospects Carlos Gonzalez and Aaron Cunningham and first baseman Chris Carter, whom they sent to Oakland. By trading for Haren, the Diamondbacks were turning future value into present value, at little cost to their chances now or in the near future.

What makes sense for one team, though, rarely makes sense for another. Arizona is a very good team facing a tight pennant race with San Diego and Los Angeles; Seattle is a mediocre team with little plausible chance of making the playoffs. (Baseball Prospectus, for instance, projects they’ll lose 89 this year.) With Bedard and Hernandez, they have an intimidating front two, but they have too many players who can’t hit in the lineup, their bullpen is weak, and the expensive back end of their rotation is surpassingly sketchy. Trading the future for the present doesn’t make as much sense when the present is looking grim.

In addition to all this, and making it all the worse, Bedard is nowhere near as valuable as Haren, simply because of his contract. He’ll be paid $7 million this year, likely $12 million or more next year, and then he’ll be eligible for free agency. He’ll be worth more than he’s paid, but not as much as Haren, nor for as long, and the wins probably won’t do the Mariners all that much good. Meanwhile, Jones will be in Baltimore, providing broadly comparable value for not 5% the money, and Seattle still won’t have a decent right fielder.

The Mariners, one suspects, fell prey to one of the great delusions of baseball: the belief in twin aces. Mets general Omar Minaya once did the same when he traded Grady Sizemore, Cliff Lee, and Brandon Phillips with visions of a Bartolo Colon/Javier Vazquez axis laying waste to the National League. This theory holds that there is nothing so valuable as a one-two punch atop the rotation, and that it can by itself cure any number of structural weaknesses a team may have, in addition to assuring good odds in the postseason, should the team get there.

In truth, though, the best teams rarely have two aces. Since the 1994 strike, only three World Series champions (the 1995 Braves, the 2001 Diamondbacks, and the 2004 Red Sox) have featured a classic, devastating pair atop the rotation. And even aside from Seattle, some of the better duos in the sport, such as Atlanta’s John Smoltz and Tim Hudson and Toronto’s Roy Halladay and A.J. Burnett, play for teams that are hardly locks for contention. Good pitchers are nice to have because they’re good pitchers; aces are even better. As the standings will show in October, though, they’re not worth selling the world for.

tmarchman@nysun.com