I did some search and got some mixed results on the subject, so here is my question:
If I install a race seat with 5 point harness, but the car doesn't have a roll cage, would that make the car more dangerous in case of a crash, or at least in case of a rollover?
can somebody explain the concept if it's true please.

 

Yes. If you don't have a roll bar or cage (and a good one, none of the cusco shit) and a harness and you roll, your upper body cannot deflect downwards to fit under the stock roll hoops (your body is held upright). Your head will be the highest point in the car, and will probably scrape along the ground (or worse, you get the point). Also, you need a proper race seat to use a harness in an S2000, because the stock seats are not designed to properly accomodate a harness. In an accident then can slip down off the side of the seat and leave you with no upper body restraint.

Basically, get a good roll bar (Hard dog, etc) and a race seat before you thing about getting a harness.