I think that's a good guess, but I'm not convinced they prevent the wagging. These bikes really like to crack license plates even with the rails on.

My personal assumption is that the rails hope to save the paint in the event of a drop or low side crash. They provide protection for the tail light wiring and a mounting point for the rear lights. By having a modular design the lights won't blow out their mounting holes in the event of a fall when they inevitably break.