I have heard that Williams is vastly overrated, a horrible cover guy, and a bad character guy.
Although the Patriots have had some major DB troubles recently, I think they're more concerned with getting stronger at cornerback than at safety. They have two young guys, James Sanders (who they just re-signed) and Brandon Meriweather, who I think they're comfortable with at safety. Meriweather was a little disappointing in the 1st year in New England, but last year he progressed nicely and was often praised publicly for his play by Belichick. I think the Patriots anticipated Meriweather being Rodney's replacement when they drafted him a couple years back, and last year while Harrison was injured for most of the year, Meriweather logged 83 tackles, 2 sacks, 2 interceptions, and 2 forced fumbles in 11 games started at strong safety.