The stock steel swing arm on the ER6n / Ninja 650R flexes under heavy acceleration and hard cornering. You can feel and hear the chain trying to run off the rear sprocket with this happens.

The Aluminum Versys swing arm is lighter, stronger and more rigid than the ER6n steel swing arm so should eliminate that problem. It should also be more corrosion resistant than the steel swing arm on the ER6n / Ninja 650R.