ACR LTT CREW QUESTIONS LEGALITY OF IAP ILS APCH AFTER RWY CHANGE AND RPTED RVR IS LOWER AFTER THE LOM; FAF.

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WHILE BEING RADAR VECTORED TO RWY 3R AT DTW; VISIBILITY WAS RPTED AS 3/4 MI WITH RWY 3R RVR 4500 FT. WE WERE THEN CHANGED TO RWY 3L AT DTW WITH NO RPTS OF LOWER RVR FOR RWY 3L. WE WERE SWITCHED TO DTW TWR ONCE PAST THE OM AND BELOW THE GS INTERCEPT ALT ON THE LOC AND GS. TWR THEN RPTED RWY 3L RVR 1400 FT. THE RWY 3L APCH PROC REQUIRES 1800 FT RVR. WE INFORMED THE TWR WE NEEDED 1800 FT RVR TO LAND. TWR THEN RPTED RWY 3L TOUCHDOWN ZONE RVR 1400 FT; MIDPOINT RVR 2200 FT; ROLLOUT RVR 2000 FT. WE CONTINUED THE APCH AND HAD THE RWY IN SIGHT AT 400 FT AGL AND THEREFORE LANDED WITH NO INCIDENT. AFTER LNDG WE EVALUATED THE SIT AND DECIDED WE SHOULD NOT HAVE LANDED BECAUSE THE TOUCHDOWN ZONE RVR IS CTLING. I BELIEVE THIS SIT WOULDN'T HAVE OCCURRED IF WE WERE GIVEN AN EARLIER RPT OF THE LOW RVR ON RWY 3L. IF WE HAD RECEIVED A RPT PRIOR TO THE OM/GS INTERCEPT ALT; WE WOULD NOT HAVE BEEN ABLE TO START THE APCH TO RWY 3L. THE DTW ATIS HAD NO RVR RPTS AND WHILE ORIGINALLY BEING VECTORED TO RWY 3R THE RVR WAS RPTED WELL ABOVE LNDG MINIMUMS. WE DID NOT RECEIVE ANY INDICATION OF THE LOW RVR FOR RWY 3L UNTIL ON THE APCH INSIDE THE OM. THIS IS THE MOST CRITICAL PHASE OF FLT AND WHEN CREW WORKLOAD IS AT ITS HIGHEST.