Sander Phillipse from over at Bucs Nation provides an excellent synopsis of the theory behind using specialists on defense, something that Pete Carroll and Dan Quinn have done over the past few years.
Some teams want generalists. They want versatile players who can fill a lot of roles. Greg Schiano did some of that: he tried to turn Adrian Clayborn into a stand-up linebacker who could rush, run and cover, his cornerbacks had to be able to work the slot, his safeties were largely interchangeable, and everyone had to be able to blitz and cover.
Other teams want specialists. They want their defensive linemen to rush the passer, and their linebackers to drop into coverage. They want their cornerbacks on the outside, and their safeties deep. They want to be able to drill specific tasks over and over again until players can execute them to perfection -- and that's where they get their edge on the football field.