4-point harnesses, 5-point harnesses, racing harnesses, or other non-stock type seatbelts are made to hold a driver in place during extreme driving situations as exist on the race track or autocross course, so that he doesn't have to use his strength to keep himself from sliding around, and can instead concentrate on driving.
They are also made to keep the driver safe in the event of an accident in a vehicle that is equipped with a rollcage. The driver's torso is kept mostly in one place by the harnesses (they do stretch a little bit normally and during an accident). This attempts to keep the drivers (helmet-clad) head from banging too hard against the "not-going-to-deflect-much-when-hit-by-a-head" rollcage. It also performs a number of other safety duties as well.
As far as safety is concerned, rollcages are made mostly for one thing, and that is, keeping the person inside the vehicle from being crushed. In racing vehicles, they stiffen up the chassis, improving handling, yadda, yadda.. that's not the point of this.. We're talking about safety here. When a vehicle gets in an accident, the rollcage (which is usually bolted or welded to the floor of the vehicle in some manner, will do its best to absorb some of the energy and prevent the foreign object (another race car, tire wall, whatever) from entering the vehicle and harming the person inside. It sometimes does a good job of this, but it doesn't offer as much safety as you would think. However, when a vehicle rolls, rollcages (if properly designed and installed) SHOULD prevent the vehicle's roof and other parts from collapsing in and crushing the occupant(s). This is where 5-point harnesses step in, and prevent the person from flying around and hitting random parts of the vehicle as the vehicle goes from shiny-side-up, to shiny-side-down, to shiny-side-sideways, to other random configurations of the "shiny side" (otherwise known as the "top" of the vehicle).
Street cars are a whole other story. When street cars get in accidents, there is not a rollcage to protect the occupants from being crushed inside. This is where the stock seatbelts shine. Since there is no hard metal rollcage to contact (usually a few inches from the drivers head), the passenger can move a little bit more inside the cabin. In event of a rollover, stock seatbelts allow the passenger(s) to slide sideways and "lower their profile" so that the collapsing roof doesn't squish them. If a person were to have 5-point harnesses (or their equivalent) in the vehicle, the harness would hold their torso up so that only their head could move (and in a very limited manner). When the roof comes collapsing down, the occupant(s) torsos are held stiffly upright, and the roof causes either a broken neck, crushed vertebrae, snapped nerves, broken back, paralyzation, or any other range of rather nasty things we don't want to happen to us.
As for rollcages, even HEAVILY padded rollcages are much more dangerous to occupants that aren't wearing helmets (aka on the street) than anything in the vehicle normally. It is strongly recommended that you not have a rollcage installed in a street vehicle unless you plan on driving around with a helmet all day long.
So in short, don't put 5-point harnesses or a rollcage in anything other than a race car, unless you intend on wearing a helmet all over the place, having a full rollcage installed with harnesses properly mounted to the cage, and a seat that is properly mounted as well. Anything less and you're asking for trouble.
Oh and here's my disclaimer for the politically correct, out to get something-for-nothing losers.. I take no responsibility for what is said here. If you get hurt, don't come yelling at me. I'm no expert, and you can't come sue me because your uncle's brother's cousin's former wife's daughter's neighbor's second-cousin's friend got a boo-boo from something I may have said. k?
