Was Sherman a great corner? Sure, no doubt. But he's no spring chicken, is rehabbing a significant injury, and you have to factor in that he played on a defense with Bennett, Thomas and Chancellor. Gotta be careful, was he great solely because of his abilities, or was he great because he played with a bunch of top teammates?
At his age, you don't want to risk finding out he's only average (assuming he returns to pre-injury form) after paying him a fat contract. Just remember Greg Hardy, besides the image problem he never did much at Dallas. Neither did Thornton, or Zach Thomas or many others.
You get free agents like Haley, top players in the prime of their career, not a bunch of old castoffs...