I am ok with a roll bar even a cage on the street.. BUT you have to have it in the right way and run the right belts..
It would suck to be killed by your bars halo because you were just wearing a lap belt and your skull slammed into your "safety" equipement..
On my 68 with the full cage I will not drive the car without wearing my full 5-point harness. If your 4 point was set back far enough you could probley get away with just a lap belt but that depends on design.
In for a penny.. in for a pound
I got a tech question just like this the other day.. I think it merits discussion.