The concept itself is newish, in the sense that roster expansion allowed teams to bring in specialty running backs for passing situations. That novelty has worn off, and because of that, the once-gimmicky "third down back" has turned more into a limited player who takes up a roster spot.
With the "three down back," that extra roster spot can be used elsewhere, instead of on that supposed limited player.
Willie Parker was thought to be a three-down back, but was often spelled by Gary Russell and Mewelde Moore. Rashard Mendenhall was supposed to fill that need, but Moore took him out in situations as did Isaac Redman. Redman was even spelled by Moore, until Moore was released, and the Steelers rotated running backs as if their roles changed possession to possession.
Now, offensive coordinator Todd Haley feels second round pick Le'Veon Bell - a 230-pound back taken in the second round of the 2013 NFL Draft - can stay on the field for three downs.
He very well could. What hasn't been publicized much about Bell is his best strength could be his pass blocking. A player's willingness as well as his ability to block the edge or free rushers up the middle is critical for playing on third downs - passing is often a priority on third downs. The combination of Bell both catching the ball and blocking so the quarterback can deliver a clean throw to another receiver is seen now as outstanding enough to think he could be on the field in any situation.
Can Bell do what his predecessors failed to do? The start of the first official organized team activities as well as this weekend's rookie minicamp will go a little bit toward answering that question.