RPTR CONCERNED ABOUT THE ACFT SIDE-BY SPACING USED AT LAX ARPT.

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DURING VISUAL APCH TO RWY 24R AT LAX; WE WERE TRACKING THE RWY 24R LOC WITH THE AUTOPLT. SOCAL APCH ADVISED THAT ACFT Y WOULD BE XING BEHIND US FOR A VISUAL TO RWY 24L. WE SAW HIM COMING ON THE IVSI/TCASII INDICATOR AND RECEIVED A TCASII 'DSND' WARNING AS HE PASSED BEHIND US EXTREMELY CLOSE AND PULLED UP ON OUR L WING AT OUR ALT AND NO MORE THAN 100 FT AWAY. HE MAINTAINED THAT CLOSE POS ALL THE WAY DOWN; EVEN AFTER I COMPLAINED TO THE TWR. TWR'S ANSWER WAS 'YEAH; A LOT OF PEOPLE HAVE BEEN COMPLAINING ABOUT THAT.' ACFT Y WAS SO CLOSE THAT HIS SYMBOL ON THE TCASII DISPLAY WAS CO-LOCATED WITH OUR ACFT SYMBOL ON THE 3 MI SCALE. IF WE HAD HIT ANY WAKE TURB; LOST #1 ENG; HAD A HARD-OVER RUDDER; OR ANY OTHER CONDITION WHICH WOULD CAUSE US TO GO TO THE L; A COLLISION WOULD HAVE BEEN UNAVOIDABLE. AS IT WAS; WE HAD TO LISTEN TO THE TCASII YELL 'DSND' AT US FOR MOST OF THE APCH; AND I HAD TO SPEND 1/2 MY TIME KEEPING AN EYE ON ACFT Y INSTEAD OF FOCUSING ON FLYING THE APCH. UNPLANNED 'FORMATION' FLT IS PROHIBITED BY THE FARS; AS WELL AS CARELESS AND RECKLESS OP; BOTH VIOLATED BY THE ACFT Y CREW. THE TWR TRIED TO BLOW IT OFF BY SAYING THAT ACFT Y HAD BEEN INSTRUCTED BY SOCAL APCH TO GET AS CLOSE AS POSSIBLE; BUT NOT PASS US. CTLR INSTRUCTIONS CANNOT SUPERSEDE THE FARS; ESPECIALLY IN A NON EMER SIT. IT IS MY UNDERSTANDING THAT SOCAL IS CREATING THESE DANGEROUS SITS; BUT THE ACFT Y CREW NEEDED TO USE BETTER JUDGEMENT TO ENSURE SAFE SEPARATION. SEVERAL DEPLANING PAX ASKED ME WHY THAT OTHER AIRPLANE WAS SO CLOSE TO US AND SEEMED VERY CONCERNED. I WAS VERY CONCERNED; SO I'M SURE OUR CUSTOMERS WERE SOMEWHAT ALARMED AT THE SIGHT OF ANOTHER ACFT THAT CLOSE TO US.