I don't care how good your rush is, a modern day QB will eat a bad secondary alive. QB's have an internal clock, if you can't cover they will get rid of the ball long before the best of rushes get there.
Getting rid of the ball and making a completion are 2 different things. If the pass rush leads to a QB throwing the ball away then that is a win for the defense. It can lead to just a bad throw, an off target throw giving the DB a better chance to defend, or not seeing a defender in that area that can pick it off. Favre was a great QB and look at all the INT's he had throwing the ball away and that's what a good pass rush does for you, it makes the QB get rid of the ball before he's ready to and before the WR is ready also.
Look at what happens when Brees gets pressured up the middle and has to rush his throws.