It really depends on the skill sets of the RBs though. If your two RBs are just pure RBs, then yeah I get what you're saying. But I'd argue that our 12 personnel with Hurd, Kamara, and Dobbs presented a lot of challenges for defenses that most other teams' 12 personnel wouldn't present.
When we had Hurd and Kamara both playing at full effort, having them both on the field was our most dangerous formation IMO. One of those guys was gonna present a mismatch for a defender and 2/3 had the explosiveness to really make a defense pay if a single guy missed his read.
Hurd' skillset basically made him like a 2nd TE who could lead block/pass block for Dobbs or Kamara and also be a threat in the passing and running game. Kamara was as shifty as any back in the NCAA last year and good enough in the passing game he could probably have played slot for a lot of teams. Dobbs of course is probably the best running QB we've ever had play here.
Because of those varying skill sets, teams had to choose whether to defend it as 12 or 21 personnel, when really the guys we had on the field were versatile enough to us to run plays as if we had both sets of personnel on the field at once.
If every team had two guys with the skill sets of Hurd and Kamara, I think you'd see a lot more two back sets nowadays. But most teams don't have that kind of talent in the backfield (or at least not two backs who are as versatile as those two were).