Considering that they must be hundreds of times more expensive than long sticks with no hinge, I would say the reason must be that they're better at threshing.
Slaves was probably the wrong term, my understanding is more like oppressed farmers as opposed to slaves?
In the end though, I'm not an expert. I'm repeating what people who seem to be experts have told me and it makes sense - but I can't judge who is an expert. (either you random hacker news commenter, or whatever other "expert"). I'll gladly stand corrected if anyone can show they really are an expert.
If you've ever been a kid copying TMNT Michelangelo with home made nunchucks you've almost certainly smacked yourself in the face.
Y'know what's martially better than two sticks with a string between them? A single big stick.