One major issue is that Benson was upset last year sharing carries with Bernard Scott, clashing with OC Jay Gruden about playing time; how's he going to feel about sharing the ball with Ridley, Vereen and Woodhead?
Cincinnati Bengals’ Cedric Benson Just Punched His Ticket Out | Bengals 101
Benson Showing Signs Of Frustration - Cincy Jungle
I also read somewhere (sorry, couldn't find the link) that he not only wanted to be the feature back, he was looking for a contract that corresponded to that role.
In addition
there is this:
His biggest downfall has been his ability to hang onto the ball. In the past two seasons Benson has fumbled the ball (12 times) nearly the same amount of times he has rushed for a touchdown (13 times). Benson has been criticized for his inability to create plays, burst through open lanes, and his inefficient pass-blocking skills.
Cedric Benson NFL Football Statistics - Pro-Football-Reference.com
In the last two season Benson has averaged 3.5 yards per carry and 3.9 yards per carry; let's not get carried away with that 1,000 yards rushing stat. He's been overused: 296 touches last year, 349 in 2010, 318 in 2009 and 258 (despite missing four games) in 2008; he'll also be 30 before the season is over. I don't think there's any tread left on those tires; running backs that get used that much tend to drop off dramatically and never recover.
I fully agree that a veteran RB should be brought into camp since Danny Woodhead is now the most experienced RB on the roster; I just don't think Benson is the right fit for multiple reasons.