All cars here in the U.S. that will be on the race track for street style racing and drag cars that go faster than 11 seconds have to have a 7 point roll cage. Any faster require more points. Now the rules may change at any time and my be different at each venue but that's avg. As for street cars they have nothing and it's illegal to drive a car with a roll cage on the streets.

If you are building a race car from scratch meaning all you have is a shell you would build in a roll cage. That would then and only then stiffen the car. It would all get welded in and become a part of the frame. But I believe what you all are talking about is taking a stock car and putting one in. This will do nothing except keep you safer in a roll, side impact, as well as a front and rear impact.