The SR strategy is one of exclusively taking the position of the interior. Being purely defensive gives extreme advantages to your attacker. They are defensive and offensive. You are not going to win a war by purely defensive tactics, you are only going to ensure that a single mistake on your part is all it takes for an attacker to defeat you. You should force your adversary to also take the position of the interior in respect to their own strategies, because then they also are forced to protect from every single possible attack, a much less pleasant position to be in than eternally waiting for one single weakness in their opponent.