RB is not a strength of this team...JJ is not a great back...he doesn't run with power, doesn't break tackles, and can't catch the ball, among other things. However, MBIII is not a great RB either.
Our RB's, though I love what they've done as a team and recognize they are doing it behind an offensive line that's average at best, are just run of the mill RB's in this league...they are not stars and never will be. They are never going to make a name for themselves, ala Don Perkins, Calvin Hill, Duane Thomas, Tony Dorsett, Hershall WAlker, or Emmitt Smith.
Julius Jones and Marion Barber III??? Nope, I don't think so...they don't have the talent.
However, they are both hard workers and will do even better behind a better OL. We made some nice trades to get Spears and JJ, but three years into it, I think it's clear we'd be better off with Stephen Jackson, at least in the short term.
Unless a top RB were to fall into our laps the way Emmitt Smith did (we weren't looking for him either), we need to focus our offensive attention on OL and WR, imo.