I too have the Reese interchangable ball (the new model, not the recalled) and love the way the ball is free to spin on the shaft. I don't see any possible way the ball could come off. Negative tongue weight is dangerous no matter what type of ball.

An important thing to remember if you leave the receiver in the hitch be sure to lubricate the pins or latches of interchangable *****. I've seen a Convert-A-Ball pin that had to be driven out with a punch and hamer.

 

One word: ROTOEZE!!! I've got the impressive tri-ball/drop setup, should I need something other than my combo 2"/pintle setup. This (rotoeze) can tackle trailers up to 10,000# and 1,200# tounge. They are some Dvmn tough hitches! Drops/rises up to I believe 6 or 7 in. Only disadvantage is they cost about $200. They sure will last you, though!