That's something that's been argued over for forever in the SCCA. The reason they allow them is to drastically lower the tire choices for competitive purposes because with street tires there are tons of tires and they change every year. With R comps there are currently only 2 competitive choices and the tires both seem to be near equally fast(I'm sure Rich G. could go into even more details about all this, or correct me if I'm wrong). Even with these tires though rollovers are still very rare.
Thorne, I would start with a 4-point rollbar that is designed so that it can be added onto later if need be, this way you can at least run harnesses properly.