If you install the harness without a functional roll cage, you pose a great risk of breaking your back if your car flips over. If your car flips over, and you have those harnesses on tight, the roof may cave in and come in hard on your head causing spinal cord injury. With the stock seat belts you would be able to tilt your body/neck/back to the side to prevent spinal injury. If you still want to install these harnesses, make sure you mount them at the same level of your shoulders or near to it.
i never had thought of that but it does make sense. very informative