Well, the conventional thinking would be that when teams are playing from behind, they have to pass more, and the defense knows it, so they can 'pin their ears back' and just think about getting to the QB.
Again, the last time the three of them stayed healthy for just 4 goddamn games in a row they combined for 26.5 sacks, if I remember correctly. When the three of them aren't all healthy, it follows that Kelsay and Denney can't rotate on the left side (as either one of them is hurt, or Schobel is hurt and one of them has to play the right side), which could keep them fresher and more effective. We haven't seen them get that chance for an extended run since 2006, and it worked out okay at the time.
I'd love to add a DE, I'm actually hopin' and prayin' for either Brown or Ayers, but there's reason to think the rush might improve without a rookie. As long as everyone's healthy, and the offense is good. We'll see.