This allows the arbor to come crashing to the floor and the pipe to go crashing into the grid. So, about those spreader plates. They are designed to keep the arbor rods from bending. With the rods all twisted, the weights could easily fall or be forced out the sides of the arbor. This is a bad thing.
Weights bouncing around the stage are a definite no-no.
The plates hold the rods by being placed into the counterweight stack properly. So just before the loaders on the bridge start putting weights into the arbor, they have to lift these plates up to the top of the arbor and tie them there out of the way. Then they start putting weights in. Then they continue putting weights into the arbor. The plates are stuck there in the stack and they hold the rods together.