When this happens, it's a necessary move that makes the Mets better immediately: Ike Davis is simply better at baseball than Jacobs. He's a better defender, and more importantly at first base, a better hitter. The Mets could have simply optioned Jacobs to the minors, but then he would still occupy a place on the team's 40-man roster. By DFA-ing him, they clearly intend to add a player, not yet on the 40-man to the big league roster in short order. The Mets now have ten days to trade or release Jacobs, or he clears waivers, send him to the minor leagues.
Davis has done exactly what a top prospect is supposed to do, hitting .364/.500/.636 in his first 10 games in AAA. He has three doubles and two HR. As impressive as the power is the way he's not just controlled, but dominated the strike zone: 9 BB and just five strikeouts. (Jacobs didn't draw his ninth walk with the Royals until May 5th last year, in his 25th game of the year, at a point when he had 27 strikeouts.)
I really think that Davis would benefit from more time at AAA, but the Mets, 15th in the NL in runs scored, can't afford to wait. According to EQA, the Baseball Prospectus metric, the Mets are the 14th-best offense in the NL. This is a team not getting it done with the bat and Jacobs and 1B has been a huge part of the problem. Mets 1B have hit .191/.269/.298 this year, placing them in the bottom three in the NL in all three major rate categories, AVG/OBP/SLG.
Jacobs has been terrible (.208/.296/.375 in 24 AB) in seven games for the Mets, but even that was fairly predictable. After all, he hit .228/.297/.401 in 128 games for the Royals last year, his second straight season playing in over 110 games with a sub-.300 OBP. What's remarkable about Jacobs start is how unremarkable it really is. He's 29 and has been a replacement level player or worse for each of the last three years.
I'm reminded a little bit of the 2007 season. The Brewers kept Ryan Braun at AAA Nashville where he hit .333/.403/.675 with 9 HR in 33 games for the Sounds. He hit .324/.370/.634 in 113 games for Milwaukee, in a year in which the Brewers missed the playoffs by two games. Would promoting Braun (and Yovanni Gallardo, who was throwing lights out in Nashville) earlier have made up the two games the Brewers finished behind St. Louis? There's a fascinating argument there. I'm not at all suggesting that Davis is going to hit like Braun. That's not fair. I am pointing out that recent teams have potentially missed out on the playoffs by waiting too long on their prospects.
This is one the Mets are getting right: putting the better, younger player in the lineup, is an easy choice.
I wanted to make one more point here: calling up Davis now, and keeping him on the big league roster all year long would make him eligible for super 2 status, giving him an extra year of arbitration. Thus, he'll be more expensive more quickly than if the Mets held him back in the minors for another month or so. Becoming a super 2 does not make Davis eligible for Free Agency any earlier.