We don't NEED five running backs. However, as analysts keep on reminding us, no running back should be expected to complete the season with injuries. More than any other position, running backs get injured. Keeping only four RB's does allow us a 9th OL. However, our 9th and 10th OL are often on the Practice Squad attending Dante's Academy. The bottom line is that carrying 5 RB's provides needed insurance at the position.