Brent Grimes missed the entire 2012 season with an Achilles injury. The only reason why most of us even think of him in any kind of heralded terms is because of his penchant for picking off Josh Freeman. Atlanta did not have particularly strong numbers against the pass between 2009 and 2011.
I know the Bucs pass defense is fricken' awful (thanks in part to a pass rush that is pretty poor, and even worse now with Bennett gone), but signing middle tier CB's like Brent Grimes isn't going to fix that. Geez, for that salary, the Bucs might as well take a flier on Ashomugha to see if they can find something in the tank after the dismal seasons in Philly. At least that's a former All-Pro who theoretically shouldn't be far removed from his prime.