A320 CREW ON THE PDZ ARR INTO LAX RECEIVE A TCASII RA JUST PRIOR TO REACHING ARNES.

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NMAC WITH ANOTHER ACFT. FO WAS FLYING. GOT A TCASII RA WHILE FLYING ASSIGNED RTE AND ALT. ATC CTLR WAS BUSY. WE WERE JUST LEVELING OFF AT 11000 FT; THRUST IDLE; SPD BRAKES DEPLOYED. WE DID NOT SEE BOGIE ACFT; BUT TCASII SHOWED HE WAS CLBING RAPIDLY INTO US. WE FOLLOWED TCASII CLB COMMAND; BUT AS YOU MIGHT EXPECT; ENGS SEEMED TO TAKE FOREVER TO SPOOL UP; AS SPD BRAKES WERE RETRACTED. TCASII SHOWED SEPARATION OF 400 FT FROM ACFT. TOLD CTLR ABOUT OUR RA; AND HE STATED THAT THE BOGIE 'JUST POPPED UP' AND HE DIDN'T HAVE TIME TO WARN US. ('JUST POPPED UP AT 11000 FT INDEED! YEAH; RIGHT!) CLR OF CONFLICT; CONTINUED TO UNEVENTFUL LNDG. SPOKE WITH TRACON SUPVR AFTER LNDG; AND HE TOLD ME THAT THE BOGIE ACFT WAS WELL WITHIN HIS RIGHTS AND COMPLETELY LEGAL IN WHAT HE DID; SINCE HE WAS OUTSIDE OF THE CLASS B AIRSPACE. ALLOWING VFR ACFT TO PASS THROUGH THE ARR CORRIDOR OF ONE OF THE NATION'S BUSIEST ARPTS; WITHOUT CONTACTING THE CTLING ATC FACILITY; IS STUPID. WE COULD HAVE LOST THIS ACFT WITH 138 PAX AND 5 CREW.