BRNAV is for area navigation not terminal navigation. Basically below FL100 your not meant to use it (but we all do). That's a bit of a surprise. Where did that come from?
This is all very entertaining but apart from M609's considered opinion, we don't seem to be unearthing the underlying benefits of PRNAV and how it is going to unlock capacity.
Mad jock (is there another sort??? - only joking! Please take it as such.) also touches on another point: increased accuracy in flying the SID can have the effect of concentrating the noise. This has already happened: where noise was acceptable because it was spread, increased adherence to the precise SID has meant that the same (groups of) houses are always receiving the whole noise benefit of jets at full chat.