There's this cliche misconception that running the ball wears out defenses. That belief was born in the days of 21 Personnel, power running games and simple passing attacks. It may still remain true for the select handful of teams that have the personnel on offense to physically bang defenders.
However, a true tempo spread attack will wear out a defense more effectively than a clock-eating run attack if it's efficient. (can't go 3 & out)
You get 230lb to 300lb box defenders running sideline to sideline chasing slot WR's and bubble screens (after rushing the QB) and you'll see how worn out they become. (quickly)
As a Linebacker I'd rather run downhill, bang a Fullback and then rest for 30 seconds than chase WR's and RB's all over the the field every 15 seconds. Any time you spread a defense out you make them run more. Anywhere from 4 to 6 players on that defense aren't huge fans of running but they have to in order to properly defend bubble screens, options, jet sweeps, RPO's, etc.
Besides, we're mainly a spread team now anyways. There's not TE or FB to bang and "wear out" defenders. It's all zone blocking.
The thing is, if you're running spread personnel, the defense is going to counter with nickle and dime personnel. You're not going to wear out the 180lb DBs chasing your slot receiver around the field. If you just want to throw it on every down, it completely eliminates any type of run responsibility for the d linemen. So the defense can just use it's four best pass rushers to get upfield as quickly as possible while playing six defensive backs behind them to chase receivers. There has to be some threat of a running game.
1. LOL You read that whole post and found one thing you could contradict, personnel. Ok, so even if a team goes Nickle that's still 6 guys who are likely 230+. (I said "anywhere from 4 to 6 defenders) And if you go Dime that means you only have 5 run defenders in the box.
2. You're assuming that the defense KNOWS you're throwing every down. (which they don't) So how could they possibly put nothing but pass rushers on the field every play? Even if that was the case, you put in four pass rushers and have them go up the field every play then guess what that does? OPENS UP THE RUNNING GAME. (and screen game)
What's with the logic that spread = no threat of a running game?