Miser - my understanding of this is that you are CONFIRMING that the RVR is AT LEAST as reported - ie that the RVR is not OVER-READING for what you actually have in the take-off path? If you see LESS than your specified minimum (due to fog banks etc - remember the transmissometers are not ON the runway) you do not go, non?
Regarding 'ignoring' RVRs, this happened at Aberdeen last month when the heavy snow had fallen, and there was a degredation of RVR readings due to blowing snow. Vis was a genuine 40k+ and I guess you need ATC to confirm a 'fault' with the RVR machine and give a 'human obs'? I also remember a problem at Edinburgh ?last year? when a fine drizzle brought the 'RVR' down to 120m when the actual vis was 10k+. Again, I think ATC 'killed' the RVR system.