Are you serious? It's NOT easy to come in for a few reps & get 3 sacks & a FF in a handful of 4th quarter plays! If it's so easy, why isn't Spence getting a ton at the beginning of the 1st quarter when HE's fresh? I don't care how close Spencer gets, or how much pressure he puts. As long as the QB is erect, there is a chance for a big play.
So what if Butler can't play the run right now. That's not what is being asked. When it's an obvious passing situation, you need to have your best pass rushers in the game...period. No way Spencer gets to McNabb on that play. How can Butler prove he can play even on tired legs when he's not given a chance to come in at all unless someone is hurt.
You make the point about fresh legs. Wouldn't that be a great reason to rotate Spencer & Butler on passing downs? That way, they BOTH have fresher legs. Right now, Butler is the next best PR LB next to DW. Putting him in on passing downs just makes sense...period.