B757 LNDG SFO EXPERIENCED MARGINAL WX DURING VISUAL APCH.

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WE WERE BEING VECTORED DOWN THE BAY FOR A VISUAL APCH TO RWY 28R AT SFO. THE FMS BRIDGE VISUAL APCH HAS BEEN LEFT OUT OF OUR DATABASE; SO WE HAD SETUP AND BRIEFED FOR THE QUIET BRIDGE RWY 28L/R APCH. ATIS WAS CALLING FOR A 5000 FT CEILING AND 6 MI VISIBILITY IN LIGHT RAIN SHOWERS. 'CHARTED' VISUAL APCHS WERE IN USE. AT 2500 FT ON A VECTOR TO THE ARPT; WE WERE STILL IN AND OUT OF THE CLOUDS AND COULD SEE THE SAN MATEO BRIDGE; BUT NO RWY OR ARPT. APCH DSNDED US TO 1500 FT AND KEPT ASKING IF WE HAD THE RWY IN SIGHT -- WE REPLIED IN THE NEGATIVE. APCH ASKED US TO TUNE IN AND JOIN THE LOC FOR RWY 28R; BUT HE DID NOT CHANGE OUR HDG. THE ILS WAS ALREADY SET; BUT THERE WAS NOT TIME TO REPROGRAM THE FMC AND REBRIEF THE APCH AS REQUIRED BY OUR FOM. AS I KEYED THE MIKE TO ASK FOR A DELAY VECTOR; I SAW THE RWY AND VASI AT 1 O'CLOCK. AN UNEVENTFUL LNDG WAS MADE. THE WX (IE; RAIN SHOWERS) WERE RESTRICTING THE VISIBILITY; AND THEY WERE STILL ATTEMPTING VISUAL APCHS. THIS PUT US IN AN UNCOMFORTABLE POS FOR A FEW SECONDS. I SHOULD HAVE ASKED TO BE BOXED AROUND FOR AN ILS APCH WHEN HE ASKED ME TO JOIN THE LOC.