Before interferometers came in RVR was assessed by humans, and this is still a valid way of measurement. A pilot sitting on the button waiting for takeoff has the accurate and overriding measurement of RVR and should act according to his reading.
The factoring system has caveats. It must not be used if an RVR is available from ATC. It must not be used for Cat ll or Cat lll ops, and not for takeoff, for the reason given above.
It is not funny to arrive in the flare on a visual approach and find you can't see anything outside. Stick to the rules, even if you can see the runway from way out it is the last few feet that count.
Dick W