The New York Jets have a defensive scheme that puts a ton of pressure on its cornerbacks. In the Playoffs we saw Gang Green play a lot of coverage, but Rex Ryan is at heart a guy who wants an attacking defense. He sends consistent blitzes. He needs corners capable of holding up in man coverage to avoid getting burned when those blitzes get picked up.
Cromartie is in this sense a good fit. At 6'2' and 210 pounds, he is athletic enough to stay with just about anybody. He is also blessed with sprinter speed. Cro ran a 4.3 40 yard dash at his pro day before the 2006 NFL Draft. In a system where corner play is considered at a premium, guys like him can be very valuable.
The Jets used Darrelle Revis in 2009 on an island much of the time and rolled coverage away from him. Adding a second corner who could be trusted on an island allowed for greater flexibility in game planning this season. Take the Divisional game at Foxborough, where the Jets trusted Revis and Cromartie to man the outside, which freed up extra players to clog the middle.