Swinging a sword might seem pointless but It actually is a great example of how your shoulder CAN move. Just like throwing a ball or in this case rotating a dumbbell it exposes the shoulder joint to its full range of motion. I'd recommend trying this with a 1kg DB first and build up with slow controlled movement to let your shoulder strengthen in both internal and external rotation. This is just one of the movements that helps to bulletproof my joints. Why not check out The Simplistic Mobility M