TFC CONFLICT ON FINAL APCH TO RWY 34R/L AT SEATTLE; WA.

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WHILE BEING RADAR VECTORED FOR A VISUAL APCH TO PARALLEL RWYS; WE WERE ASKED IF WE HAD OPPOSING TFC (BEING VECTORED TO OTHER PARALLEL RWY) IN SIGHT. WE REPLIED WE DID NOT; BUT WE HAD THE RWY IN SIGHT. THE CTLR THEN CLRED US FOR THE VISUAL APCH WITH THE REMARK; 'THE OTHER ACFT HAS YOU IN SIGHT AND WILL MAINTAIN VISUAL SEPARATION.' HALFWAY THROUGH THE TURN FROM BASE TO FINAL I SPOTTED THE TFC AND TOLD THE CTLR; 'WE SEE THE TFC; BUT WILL LOSE SIGHT OF HIM AS WE CONTINUE THE TURN.' (WE WERE 'BELLY UP' TO TFC AS WE CONTINUED THE TURN.) AT THIS POINT IT SHOULD BE NOTED; THAT WE WERE WELL ABOVE GS DSNDING AND THE OPPOSING TFC WAS BELOW GS FLYING INTO THE GS. 5-6 MI FROM THE RWY; THE TCASII GAVE THE VERBAL ADVISORY; 'TFC.' A FEW SECONDS LATER; AN RA; 'CLB' WAS GIVEN. SHORTLY THEREAFTER A 'DSND' ADVISORY WAS GIVEN. DURING THE 'ADVISORY PERIOD' WE WERE APPROX 200-300 FT R OFF CTRLINE; GIVING THE OTHER ACFT EXTRA ROOM FOR THE TURN-ON. AFTER THE FIRST RA; I SPOTTED THE OTHER ACFT OFF THE L SIDE; VERY CLOSE TO US. MY BEST GUESS 300 FT. IT WAS APPARENT HE WAS TRACKING TO THE R OFF CTRLINE FOR HIS RWY AND DIRECTLY ABEAM US. AT THIS POINT HE STARTED TO SLOW; AND FALL BEHIND US THE REST OF THE APCH. IT'S READILY APPARENT EITHER HE LOST SIGHT OF US (AND DIDN'T RPT IT TO ATC); OR OVERSHOT FINAL AND ACCEPTED TRACKING R OF COURSE OR BOTH. THOSE OCCURRENCES HAPPEN. THIS ENTIRE SIT COULD HAVE BEEN AVOIDED IF: ATC HAD APPLIED A 'STAGGER' TO OUR SPACING ON FINAL; OR IF THE RULES WERE CHANGED WHEREBY BOTH ACFT MUST HAVE EACH OTHER IN SIGHT THROUGHOUT THE ENTIRE PROC. THERE'S TOO MUCH RISKED DEPENDING ON ONLY 1 ACFT'S ABILITY TO MAINTAIN VISUAL SEPARATION (EACH OTHERS) (TO REACH THE GS); ALSO; HAVING TO 'CROSS' ALT; WHEN ABEAM SPACING IS USED WRONG. PERIOD. SUPPLEMENTAL INFO FROM ACN 489631: STARTED GETTING NUMEROUS RA'S WHILE TRYING TO GET DOWN TO GS. CHOSE TO IGNORE BECAUSE FELT CLOSE PROX TO OTHER ACFT REQUIRED TO KEEP HIM IN SIGHT; TURNED SLIGHTLY E TO LINE UP E OF RWY 34R RWY EDGE. REMAINED OFFSET UNTIL WE PULLED CLR. CALLED ATC UPON BLOCK IN. ASKED TO FILE MIDAIR. SLAM DUNK; HEAD TO HEAD TURN BY ATC EXASPERATED AN ALREADY HAZARDOUS CONDITION (CLOSELY SPACED PARALLEL APCH) INTO A NEAR FATAL ONE. ATC WILL BLAME ACFT Y; BUT ATC SHOULD BEAR THE MAJORITY OF THE BURDEN. SAFETY TOOK A BACK SEAT TO CAPACITY.