PLT ON B737 RPTS SIGHTING AN MD80 ON APCH AT SFO FOR THE PARALLEL RWY. PLT IS UNABLE TO SEE THE ARPT DUE TO REDUCED VISIBILITY BUT IS CLRED FOR VISUAL APCH. PLT BECOMES UNCOMFORTABLE WITH THE SIT AND EXECUTES A MISSED APCH. THE FO BELIEVES A NMAC OCCURRED BTWN HIS FLT AND THE ACFT HE SAW AHEAD FOR THE PARALLEL RWY.

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OUR FLT WAS ASSIGNED A HDG OF 140 DEGS FOR A L DOWNWIND APCH FOR RWY 28L AT SFO. WX WAS APPROX 190 DEGS/20 KTS BROKEN 020 5 MI VISIBILITY WITH LIGHT RAIN SHOWERS OVER SFO AND MOSTLY CLR OVER BRIJJ; THE OM FOR RWY 28L. TURNING BASE FOR THE APCH I NOTICED TFC DIRECTLY BELOW US PROCEEDING TO THE ARPT THAT WAS NOT ON TCASII. I INQUIRED BAY APCH AND THEY NOTIFIED US IT WAS AN MD80 PROCEEDING TO RWY 28R AND CLRED US THE VISUAL APCH FOR RWY 28L. WE RESPONDED BY SAYING WE HAD THE TFC AND THE BRIJJ IN SIGHT; BUT NOT THE ARPT. WE WERE GIVEN THE VISUAL APCH. PROCEEDING TO BRIJJ; WE ELECTED TO STAY TO THE L OF COURSE TO PROVIDE MORE DISTANCE BTWN US AND THE MD80. THE MD80 APPEARED TO HAVE OVERSHOT THE LOC AND WHILE TAKING CORRECTIVE ACTION TO RECAPTURE IT; OVERSHOT IT AGAIN FLYING INTO THE RWY 28L LOC. THE CAPT WAS VERY BUSY TRYING TO FIND THE ARPT THROUGH THE RAIN SHOWER THAT HAD STARTED OVER THE ARPT AND REDUCED VISIBILITY. IN ADDITION HE WAS ALSO WATCHING THE MD80 TO DETERMINE THEIR LOCATION. I WAS CONCERNED ABOUT THE FLT PATH OF THE MD80 AND WAS NOT ABLE TO DEVOTE MY FULL ATTN TO THE FLT PATH OF OUR ACFT. NOTICING THE INSTS DURING A QUICK XCHK I NOTIFIED THE CAPT. HE DID NOT HAVE THE ARPT; WE WERE L OF COURSE AND PROCEEDING BELOW GLIDE PATH. MEANWHILE; THE MD80 WAS PROCEEDING TO INTERCEPT THE RWY 28L LOC (I PRESUME) AND I FELT VERY UNCOMFORTABLE WITH THEIR CLOSE PROX TO OUR ACFT. I SAID WE SHOULD GAR AND THE CAPT QUICKLY ACTED. I ESTIMATE WE CAME CLOSE TO 400 FT TO THE MD80 HORIZLY AND EVEN VERTLY. AFTER I NOTIFIED THE TWR OF OUR GAR; I NOTICED THE MD80 TURN IMMEDIATELY BACK TO THE R. WE PROCEEDED WITH A NORMAL ILS APCH TO RWY 28R AFTERWARDS WITH NO FURTHER INCIDENTS. THE CONTRIBUTING FACTORS WERE: WX INCLUDING STRONG WINDS OUT OF THE S; AND DETERIORATING VISIBILITY ON THE APCH. ATC TRYING TO DO VISUALS UNDER QUESTIONABLE CONDITIONS (WX) AND HIGH TFC LOAD. HUMAN FACTOR ERRORS BY BOTH PLTS FOCUSING ON THE TFC INSTEAD OF ASSIGNING SPECIFIC TASKS FOR EACH TO DO AND CONCENTRATE ON FLYING THE ACFT. IN ADDITION; BY FLYING TO THE L OF THE LOC INSIDE THE OM; WE WERE VIOLATING OUR SOPS. HUMAN PERFORMANCE CONSIDERATIONS: WE WERE CLRED THE VISUAL APCH OPERATING INTENTIONALLY CLOSE TO ANOTHER ACFT WITH ONLY THE BRIJJ IN SIGHT. OVER BRIJJ; WITHOUT THE RWY IN SIGHT; WE SHOULD HAVE EXECUTED A GAR. BY NOT STATING WHO WOULD WATCH THE TFC AND WHO WOULD FLY THE APCH WE FAILED DELEGATING TASKS. HOWEVER; AFTER RECOGNIZING THE SIT HAD DETERIORATED RAPIDLY; WE DID EXECUTE PROMPT EVASIVE ACTION WITH COORDINATED TEAMWORK. LASTLY; I BELIEVE THIS IS A PERFECT EXAMPLE WITH THE CURRENT SIT IN SAN FRANCISCO. DUE TO THE LACK OF SPACE TO ADD RWYS THE ARPT IS SATURATED WITH DELAYS WHEN CONFRONTED WITH WX. WE WERE; IN FACT; 3 HRS LATE. I BELIEVE THEY WILL TRY TO 'HELP' PREVENT DELAYS AS LONG AS POSSIBLE BY CLRING PLTS A VISUAL APCH. AFTER OUR APCH AND SUBSEQUENT GAR ATC DID SWITCH BACK TO THE ILS RWY 28R APCH. I BELIEVE THIS SHOULD BE OF GREAT CONCERN THROUGHOUT THE ATC SYS BUT NO PLACE AS SIGNIFICANTLY AS SAN FRANCISCO.