Primary backup?
I'm not sure that term even applies any more. I don't think there are a lot of RB's in this league that spends almost an entire game sitting on a bench waiting in case another RB gets hurt. Of course, this could be a simple case of semantics because I remember a time when there were RB's that were there simply as a backup
I don't even think there is enough room on a team for three activated RB's unless one of them is also a special teams dynamo or has some other skill.
These days I feel that a "starting" back is only called that because he is the first to get carries during a game. At some point he will be taken out and the "2nd" back will go in. he's not exactly a backup. If either one gets injured then it means the other back will take the rest of the carries. if a 3rd EB is activated, then he will get some carries. However, in order to remain effective, the running attack will only give that 3rd RB more than a couple of carries if he is effective or if he has a particular skill compatible with a set of plays.
I think that OC's look for sets of two RB's, one will get a few more carries while the other is there because there are a few things he can do better.
For example, the idea of having a duo composed of a power runner and a slasher means you can design plays for either. You rarely find both attributes at an elite level in the same RB.
Getting back to the question, Jones could be that 2nd back. However, it ii a counterexample of what i just mentioned, diversity in skillset. Pollard and Jones are too similar, it would be like the opponent is gameplanning for one RB. The Cowboys need a bonafide power runner for redzones and 3rd and short. They need a 4th quarter closer.
