Brakes probably wouldn't ever be a problem, but a cage can be a danger to the driver is no properly belted in (especially when unhelmeted) and potentially to other cars on the road in a crash. Of course, these things didn't stop me from driving my old car for tens of thousands of miles on the street with a roll cage... However, for the reasons listed above, I can picture a situation where an (overly) strict auto inspection agency would fail a caged street car.