Good question.
YPC does not mean much to me. YPC is boosted by long runs which are great, but the primary job of the RB in terms of rushing is much more about situational performance than total yardage or ypc.
The ypc numbers also don't mean much because short yardage situations decrease ypc. You could have the greatest short yardage RB ever, and his ypc would be low.
Some type of Success Rate definition would be better than ypc.
The following could be the stats for 2 RBs on 1st-and-10. The one with the lower average is likely to be better.
RB1 ypc = 7 (80, -1, -1, -1, -1, -1, -1, -1, -1, -2)
RB2 ypc = 4 (4, 4, 4, 4, 4, 4, 4, 4, 4, 4)
RB2 consistently keeps his offense in a good down and distance for 2nd down while RB1 keeps his offense in a bad position except for 1 time.
The top scoring offenses generally have about 400 total yards with about 300 passing and 100 rushing. A 20% improvement in rushing yards would be 20 extra yards while a 20% improvement in passing yards would be 60 extra yards; therefore, the focus of rushing should be on converting 1st downs and scoring on short yardage while passing is more focused on racking up yards. Rushing will also help passing by forcing pass rushers to defend the run, forcing defenses to limit snaps for smallish pass rushers in favor of run defenders and playing a Safety in the box instead of in pass coverage. Rushing also helps with intangible issues like wearing down the defense physically and emotionally.
Having said all of that, it becomes too complicated (for fans) to define define it statistically; therefore, we just have to go by watching the games to rate the RBs.
1. EE
2. Bell