with a standard seatbelt you are gonna fly around and you are limbs and the rollcage are gonna go what the abbo's call the 'whirlwind date'.
makes sense.
You can get a roll cage like in the GT3RS, not a "full" one but a half one which goes across over where the back seats are and isn't as awkward as a full one (which is stupid on a car which is road legal).
Klutch you don't need a roll cage you have plenty of absorbing material on your person. 8)