One line of thinking is that a team's third cornerback or third safety is like another starter. The reason is that with offenses spreading the field as often as they do, defenses are using sub packages -- which bring an extra defensive back onto the field -- more often.
Here is the Patriots' breakdown this season (including penalties):
Sub defense: 159 of 272 (58.4 percent)
Base defense: 109 of 272 (40.0 percent)
Short yardage: 4 of 272 (1.5 percent)
The only game in which the Patriots were in their base more than sub was Week 2 against the Jets. Against the Bengals and Dolphins, the Patriots were in the base for the majority of first-half snaps before playing more sub defense as the score dictated in the second half.