A lot of factors have to come together. If the #1 read on a play is open, then it is easy to get rid of the ball. If you have to go through your progressions a second time, because nobody is open, then there is a pretty good chance you will get hit-whether it is a sack or not.
The threat of Spiller taking the ball the distance hopefully will keep defenses honest. Not only does he have the speed to get to the corner, he also has a good burst to get through the hole and get 4-5 yards. I also think he is much better at picking up the blitz than Lynch and Jackson.