today, a team needs three CB's just to start the game [in a passing situation]. given nfl injuries, having a solid 4th CB is a given. then, 2-3 backups are needed as well. it's like DE. for me a lot of this relates to the still absurd 53/45 man roster rules. you have 22 positions, plus P, K, and LS. that's 25 guys if no one gets hurt. 53 / 45 just aren't enough unless you want to have starters on the ST's all day long. and with fatigue and injuries, THAT would diminish the quality of play.