You use one shoulder at a time so the other gets a rest. And if you go shirtless to keep cool, with two straps neither shoulder ever gets a break from the straps chafing. With this you're resting one shoulder all the time.
Also I like the look of where it sits, in the curve of your lumbar spine rather than high on your shoulders. Seems like the weight is held less eccentrically to your core. It may not hold much but that's kind of the point of going lightweight.