More to the point, it very dangerous to use a harness with out a roll cage. if you roll ya car with a seat belt on, your body will slide down whilst bein held back with a seat belt as the roof caves in, but if your in a harness and the roof caves in, your held firmly in your seat, you can wave good buy to your neck and spinal cord. Obviously a roll cage would prevent the roof caving in.
There was a law, not sure if it still is, that even if you have a harness fitted and dont remove the seat belt from the car then you can get a fine for not wearing your seat belt, even if you wear the harness.
Also harnesses are actually a real pain in the ar5e at juntions etc.