No single unit can successfully carry an entire team on its own on a weekly basis in the NFL. Some teams may have a much better offense than defense, or vice versa, but there will be a time in almost every victory in which key plays must be made in every phase, including special teams.
It gets at the root of the concept of complementary football that Pittsburgh Steelers head coach Mike Tomlin brings up so frequently. Does the defense do what the offense needs it to do? Does the special teams do what the defense needs it to do?