The benefit of streamlining for an OW diver is reduced air consumption. The more surface area outside the "slipstream" of your body, the more drag it creates and the more force it takes to overcome that which translates into additional air usage.
This is especially noticeable in heavy currents where the way a 40" hose loops off the tank causes it to vibrate or "sing."
For some people air consumption may not be an issue, but I found that my air consumption greatly decreased when I learned how to properly streamline my gear.
It's amazing how how much your "air consumption" decreases when you start using "streamlined" CCR's