I keep going back to BADOL's train of thought -- it's not so much the players the Bills have drafted, but the continuing roster churn. As you pointed out, Pat Williams' departure left a gaping hole on the DL. If they'd kept him, they wouldn't have had to reach for McCargo.
Ditto for the revolving door at CB, which traces all the way back to the Super Bowl era: Derrick Burroughs (career-ending injury), Nate Odomes (FA-Seattle), and JD Williams (bust) begat Jeff Burris (FA-Indy) and Thomas Smith (FA-Chicago), who were replaced by Antoine Winfield (FA-Minnesota) and Nate Clements (FA-SF), with Leodis McKelvin (??) the latest rental. And except for Odomes (#2-1987), every single one of those players was a first-round pick.
Same at RB; since 1997, Antowain Smith, Travis Henry, Willis McGahee, and Marshawn Lynch (three first-rounders and a second) have all tried to fill Thurman's shoes. None of the first three lasted more than four seasons here; we'll see about Lynch. And who was the starter at the end of last season? A Division III undrafted free agent.
Meanwhile, between 1995 (Ruben Brown) and 2009 (Eric Wood), they used exactly ONE first-round pick on the OL, and missed on that one. Good grief -- no wonder we haven't seen a playoff game here since it was still called Rich Stadium.