The problem with grading a FS on eye test alone is you only ever see the absolute worst on camera come game time. Go look at the All-22 and you find out that Marcus Williams is shutting down the deep passing game week in and week out, all by himself most of the time.
This. He usually plays so deep you can't see the impact he's having. I can live with a few missed tackles from my FS as long as he's excelling at his primary job.