BRNAV is for area navigation not terminal navigation. Basically below FL100 your not meant to use it (but we all do). SIDS and STARS will all reference a radio aid if you do a PRNAV SID and STAR the points can just be abstract points in space with no reference to a radio aid.
Thats my understanding on it anyway.
Without PRNAV they have to expect that the aircraft will deviate to the max of the allowed instrument error so each nav aid will have a one dot cone coming out of it which is the max deviation. Then from the edge of that cone they then seperate to the max edge of another cone.
Which is actaully where some noise planners fall foul because they prescribe routes which are outside the nav tolerances of the aircraft.