I am no safety expert, but this is my thought process.
I had an empty utility trailer come off the ball once. On the interstate. The safety chains were properly connected, and what could have been a BIG problem was a minor inconvenience. The trailer stayed connected without dragging the tongue. I pulled to the shoulder and reconnected. The hitch latch had come open. I learned. After that, the hitch latch was always padlocked closed. No more issues.
Bumper hitch trailers have many "weak" points, including the hitch, as in my case, or an entire receiver pin shearing. This type of trailer vary in size and weight from well under 1000 lbs to well over 10,000 lbs. Regs must cover all of these. Safety chains can save your lunch!!! I have no statistics on how often this happens, and many occurrences will go unreported, like mine, because the chains did their job.
FW hitches of any type, IMHO, are much less susceptible to coming loose. Commercial tractor trailer rigs use a FW connection, and chains are not required. Again, I have no stats, but I do not remember a case where one came loose unless as a RESULT of an accident. More often, they stay connected and prevent the accident from being worse!
On my Andersen Ultimate hitch, there is a solid aluminum block attached to the FW pin, which has a ball receiver machined into it. This is firmly attached to the King Pin with high grade bolts. Once the FW is lowered onto the ball, a pin slides in under the ball, locking the ball in the aluminum block, and this pin is latched in place. Also, there is over a ton of downward force holding everything in place. (pin wt) If the ball goes into the receiver, and is latched in place, it is NOT coming off, thus no need for chains. Even if it did come off, the front of the FW would hit the pavement, stopping its travel in a very short distance, and probably keeping it upright, since the entire front edge would provide lateral stability. Chains not needed. My opinion.
