Agree with TbombZ. Keeps the hands/grip free and you can also alternate shrugs from side to side. Not too many people take advantage of this trap movement.
There is a exercise, rarely used any more, that hits the traps well. That's the Hise shrug. Having the bar on the shoulders, like getting ready for squatting, just shrug the shoulders up as high as possible. Going to need a towel or some other form of light padding, for some guy's. Can work up to some impress weight this way. The Hise shrug, back in the day, was noted as a growth exercise when higher reps (20 or more), with deep breathing, was preformed.
Something to be said for just holding a heavy weight in place, which affects the joints, tendons & ligaments in a good way. Remembering that your only hold, and not move, that extra heavy weight. Example being BB squats, DL's, overhead presses and top position benches. Even the middle position (90 degrees) of a BB curl. Good Luck.