There's another fallacy of a "bend but don't break" defensive philosophy - time of possession. If our defense is on the field, our offense can't score points (the defense must).
On a side note: I noticed that Indiana, which I believe has the 8th best defense in the Big Ten, employed a "bend but don't break" defense against Iowa. How much of a "bend but don't break" defense is the defense really responsible for?
With Northwestern's spread offense against Iowa's "ole" pass defense, it should be very difficult for them to have any time of possession advantage. Dropped passes stop the clock. Most of their offensive plays are geared to gain at least 10 yards, a first down, which also stops the clock.