Gregory Shamus/Getty Images
In the simplest sense, Chris Johnson is a speed back, and Shonn Greene is a power back. Ideally, you use your power back for short yardage situations and your speed back for open field stuff.
However, you can't just assign roles that simple to your players, or your calls become too predictable. So, you have to mix it up to keep defenses guessing.
The Titans, for a while, did that against the Colts, and it seemed to be working fairly well. They occasionally ran Johnson up the middle, and they occasionally sent Greene to the edges in rushing plays.
Now I don't think the balance was perfect, but at least they looked like they were getting close. Using two backs interchangeably can be a good way to keep both relatively fresh, and if I'm the Titans' coach, I want Johnson fresh for as long as I can have him.
Johnson won't have any breakout runs when he's more tired than the defense is. If they continue to work on balancing the rushing attack, they might finally get the kind of run game they've wanted all season.