Ptkay: You are speaking about METAR which is issue every 30 min.(normally), we are speaking about instantaneous RVR readings that are
available to the controller every 15 sec., BKN001 doesn`t constitute a limitation for ILS precision app.,Just to give an example: ATIS info A at 0200UTC RVR27 400m BKN 001, once in contact with TWR they tell you RVR now is 600/550/700 what do you do?He will not give you cloud base!
METAR and TAF are used for planning pourposes on the ground or inflight replannig!