First some reasons to bring him in.
Have a look at some video:
Clearly he can play, that is not in dispute. He has a great first step, and hits the hole like the best of them. His speed North/South is first rate. Cedric Benson is an effective running back.
And the stats bare it out.
With the Cincinnati Bengals, he finally got his chance to be the full time lead back in 2009, and rushed for 1,000 yards in every season since he took the reigns. His best season came in 2009 when he only played in 13 games, yet rushed for 1,251 yards. He has a nose for the endzone, with 19 TDs over the last 3 seasons.
He is still in his prime at 29 years old. The Jets have only one proven running back on the roster, Shonn Greene. The others are still unproven, Joe McKnight, Bilal Powell, Terrance Ganaway, etc. You could make the argument that you never have enough running backs for the system the Jets want to run.