1
when your 80 unit is the defensive backs, the intimidation factor falls much less on the quarterback than it does the receivers and sometimes the running backs. The QB may fear an interception, but even if it happens his body is still intact. When your 80 unit is linebackers and defensive linemen, the QB literally fears for his physical safety. Those Chicago Bear defenses you refer to were completely stifling in a way nobody other than perhaps the great Pittsburgh Steelers defenses. Both of those teams played in a different era, though, and the passing game was not near as dominant then as it is now.