Well, no-one said that roll-bars save weight. That would just defy all logic. Rather like putting a couple of sand bags in your boot, claiming they save weight.
And I can see why putting roll bars in a road car is just utter stupidity. Apart from the fact that they would have basically no effect on your driving (In any modern-ish car, the car is designed to be crashed by the manufacturer, so it's perfectly safe in a crash), there is the probability of banging your head off a bar (unless of course you're wearing a well-installed 6-point harness and have proper bucket seats), which would make you more of a hazard to other road users too...
In short, just don't do it