2006 - Tye Hill started 10 games for the Rams and Antonio Cromartie, in his 2nd year, is one of the top CB's in the league. Jonathan Joseph for the Bengals started 9 games as a rookie.
2007 - Revis for the Jets, Leon Hall for the Bengals, and Aaron Ross for the Giants all started and contributed last season to their teams.
I do agree that adding Gay, Young, and David in the last 2 off seasons has made a roster spot at that position hard to secure, but I don't think it's a lock to say a rookie could not contribute to our Defense.