I think manager's trying this strategy will get surprisingly "lucky" with RB value later in the draft than some might think. Even in a 12 team league, only the top 22 backs could even possibly be picked, with the one guy going WR/WR. A pretty good number of teams roll with the RBBC, so that adds a both a few RBs that are servicable AND a few backs that will drop because not many managers will spend a top 4-5 pick on a guy sharing carries.
We know at least 1 QB will go (too) early, Brady, Brees, and Palmer rarely make it out of the third, half the managers in the league will fill the starting spots, at least most of them, before picking up the 3rd back. All this leads to RBs sliding. It will happen. As long as you don't care which ones you get, you can have some combo of Green, T Jones, A Peterson, Jordan, MB III, J Jones, (name a RB with question mark) and you can pick them in or after round 5.
Personally, I like grabbing one elite RB in round 1 because I'm not too dumb, and then letting those RBs drop to at least round 4 before I get #2. It usually works well and besides that, how many times are sitting there with 2 RBs after two rounds and another RB you had eyed drops to you in round 3? 4? 5?