As a weightlifter it is my opinion that the behind the neck rack jerk is a stupid useless exercise. People do them because they are significantly easier then the traditional rack jerk. More weight can be used (in some cases a significant amount more) because the struggle to keep the weight over mid-foot is significantly diminished (this is the part where most people falter and need to practice) and the "feel" for the bounce is completely different. Also re-racking a heavy weight on to your delts is a pain in the ass. The exercise gives a false sense of security to the lifter in what they are actually capable of jerking and if done enough can interfere with the feel for the bounce.
During a behind the neck press you are subjecting the shoulder, in a compromised position, to the load through out the entire ROM. During a behind the neck push press/jerk/snatch balance the shoulder is only being subjected to the load after it's out of the danger zone. If your doing a jerk/snatch balance correctly the shoulder will receive the load in it's most stable position, regardless of starting position, making both exercises safe.
As far as re-racking goes it is my opinion from experience it is significantly easier to re-rack a bar on to your traps then to your delts. I doubt there are any statistics on this but I think you are much more likely to fracture a wrist then a vertebrae with those two injuries being the worst case scenario of each case. As I stated above this is probably one of the main reasons (even if never stated) they are so popular.