To answer the question of why the Mets should sign Elijah Dukes its easier to say why not? The Mets have a projected starting outfield of Jason Bay, Angel Pagan, and Jeff Francoeur. Carlos Beltran is on the shelf for who knows how long and has been oft injured the last few years. Dukes is 25 years old as of Opening Day, he's got some pop and speed and has shown some flashes of being a solid major league center fielder.
Lets compare him to some of the Mets other options in batting average, OBP and Slugging %.
Player A- 280/369/451
Player B- 281/337/445
Player A is Dukes while Player B is Mets current super prospect Fernando Martinez. Those are their combined minor league stats. Why not take a flyer on Dukes and and leave your best positional prospect in the minors for more seasoning.
Player A- 242/349/421
Player B- 281/331/443
Player A is Dukes again while Player B is Angel Pagan, the projected starting center fielder while Beltran recovers from surgery. Lets throw in that Dukes has hit twice as many dingers in the Majors as Pagan in only 70 more AB's.
Player A- HR every 26 AB's
Player B- HR every 31 AB's
Player A is Dukes while Player B is Jeff Francoeur. Add in that Dukes has more career stolen bases than Francoeur in one third of the games and he has arguably twice the plate discipline that Francoeur does.
So Dukes compares favorably to most of the Mets options available and he can play any of the outfield positions. Even when Carlos Beltran returns he can become a super sub that provides hitting and speed off the Amazin's bench. Yes he has had character issues in the past but his upside is worth the risk. After all Major League Baseball is a business and character flaws have been known to be ignored. How else would Ron Washington and Milton Bradley still have jobs.
Overall this year the Mets pitching staff looks quite a bit shaky and they will need the offense to carry them quite a bit, especially in Citi Field. Sign Dukes and add a young player with a good upside and give the Mets some insurance for Beltran and buy some time for Martinez to perfect his craft. Maybe even improve your outfield depth and be able to swing a deal for a decent starting pitcher at the deadline if the team is within striking distance. Worst case your fan base at least feels you're putting your best possible product out there on the field and after an offseason where Omar Minaya is under fire for his perceived lack of effort maybe signing Dukes shows us that he is paying attention to the baseball world.