Quote Originally Posted by speedminded
ANY harness in a car without a roll bar or roll cage is dumb. Period.

You roll over or get t-boned hard you are roadkill. Ordinary seat belts allow a certain amount of freedom so you aren't 100% stationary in the event of an accident, a harness doesn't allow that movement and you are a sitting duck waiting to get squished.
Actually, most modern seat belts are designed to keep you pinned to your seat. They even come with belt tensioner's that will take up slack(usually about 6") and lock within a fraction of a second upon impact.

Modern cars rely on crush zones(in conjunction with steel framework) to absorb the majority of the shock caused by an accident. The seat belt is designed to keep you stationary and the air bags are designed to fill the space between the dash or steering wheel and the passenger(s).