A torsion bar is round & you put a preload on it to get the spring effect when it twists. The stress from twisting is spread out around the bar & is evenly distributed. When stress from twisting encounters a gouge in the metal, not just a light surface scratch the stress is distributed unevenly & can cause a stress fracture leading to a failure of the bar. That is why when stress releiving something such as a block, all edges & corners are rounded off, so the stress is distributed evenly & it doesn't stop at a point that could cause a fracture.

Makes sense to me.
Oh, and so you all know, you can have the adjuster bolt all the way out and even if nothing is rusty, the torsion bars hex can and will be a pisser to separate. The grease in the socket on reassembly will reduce the efforts in the future.