Have you read about potential safety issues in the event of a roll-over with harnesses and no cage:
"there's the whole issue of harnesses without rollover protection. If your car rolls, and the A pillars collapse, you have no where to go - the harnesses keep you stuck in position,so your head and neck end up supporting the weight of the car. Not good. If you're wearing your normal 3 points, you end up pushed over sideways into the passenger seat."