This is just my personal opinion, but I wouldn't have paid $12 million for a short yardage back. Now if he was running like Jamal Anderson, I would be totally fine with that.
I think the idea was to avoid having Kamara do all the dirty work and keep him from getting so beat up. So, in a sense it's more about allowing Kamara to do what he does best, but still have someone you trust to get that 4th and 1 when the game is on the line.