I've noticed that some teams (esp in NFL) tend to use specialized RBs: James Cook is the primary ball carrier (PBC) for the Bills, but Ray Davis or Ty Johnson are on the field when the Bills are facing a 3rd down. Jax uses Chris Rodriguez as the goal line back, instead of Tuten, the PBC.
I guess there are only so many NFL "ball carrier" quality RBs to go around, and with small rosters, the HC doesn't want to burn the ball carrier's fuel by having him run routes to nowhere.
But still. It seems to be a major tip off to the D as to what is coming. And isn't that a big part of the battle? The other thing is that it kills your ability to go up tempo, because if the O substitutes, the D gets to substitute. (Not that NFL teams have the youth and roster to go up tempo very often.)
I like the "interchangeable" RBs approach. Which is where Ucla is this year. Each of the 3 guys getting reps are accomplished ball carriers. And they are all competent in the pass game. A huge Q for every college back is their ability to pass block an NFL defender. It's one thing to pick up a blitz from a Poli Sci major from Purdue. It's another thing to block a blitzing Ray Lewis (pro tip: watch his knife hand). Getting a chance to play on passing downs gives the Ucla RBs film for NFL scouts to analyze...I think it can only help them catch on at the next level.
I like it that the opponent has to respect the run game at all times, no matter which of our 3 RBs is in the game, and no matter what the down and distance is. Ucla's guys are so quick, that a simple dive up the middle coupled with some down blocks and a pulling guard can pop for 20 and convert on a 3rd and long play. Meanwhile, if Ucla goes play action on 3rd and long...the LBs have to respect Wayne Knight's (or Woods' or Thomas') run fake, which opens up the middle of the field for Rowe or Morgan to run a deep in to move the chains.
Using specialized backs takes away that element of surprise. Chicago is one of the teams that uses interchangeable backs, and that gives Ben Johnson some leverage. Fumble the ball? You miss a rotation or two. Fail to hit the hole? Coach can go with the hot hand. I think Chez is in the same position with this Bruin roster.