[Colonel Kataffy]

The only problem with your reasoning when it comes to CBs is that it fails to take into account the the interdependance of the different components that make up a defense. You are right, that if a team has a weak dline and weak linebackers, then it is gonna need amazing corners. Alternatively though, if a team has weak corners, it can be made up for by defensive ends that can put more pressure on the QB or on lineman and line backers that can stop the run alowing the safteys to help in pass coverage and so on...