COMMUTER ACFT ON VECTORS FOR VISUAL APCH PASSES SMA ON APCH FOR PARALLEL RWY.

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DURING VECTORS FOR VISUAL APCH TO PBI RWY 9L; WE WERE TOLD TO LOOK FOR TFC THAT WAS IN RWY 9R CLOSED TFC PATTERN FOR RWY 9R. WE LOOKED FOR THE CESSNA; BUT DID NOT SEE HIM; AND ADVISED APCH CTL THUSLY. WE WERE CLRED FOR THE VISUAL AND SWITCHED TO PBI TWR. AS WE CONTINUED OUR FINAL APCH TO RWY 9L; WE DID NOT SPOT THE CESSNA UNTIL AT APPROX 500 FT. THE TWR ADVISED 'THAT TFC OFF YOUR R WILL BE TURNING FINAL TO RWY 9R.' AT THAT TIME; I SAW HIM AT OUR 2 O'CLOCK POS; IN THE LNDG CONFIGN; TURNING BASE TO FINAL; WHICH PUT HIM PARALLEL AND VERY CLOSE TO US. THE SPACING BTWN THESE RWYS IS MINIMAL; AND I SERIOUSLY CONSIDERED COMMANDING A 'GAR;' BUT DID NOT. SINCE OUR APCH SPD WAS CONSIDERABLY FASTER THAN HIS; WE PASSED HIM ON SHORT FINAL; PRIOR TO MAKING A NORMAL LNDG. MY FO AND I DISCUSSED THE UNUSUALLY CLOSE SPACING AND THE POSSIBILITY OF A GAR WHEN WE FIRST SAW THE CESSNA. I BELIEVE THAT THIS WAS TRULY MINIMAL SEPARATION. I KNOW THAT THE FAA HAS BEEN VIOLATING PLTS FOR PASSING ACFT. ON PARALLEL VISUAL APCHS; AND THAT WAS PART OF MY THOUGHT PROCESS -- BUT EXECUTING A GAR IN THIS SIT WOULD PROBABLY HAVE BEEN A GREATER HAZARD. SAFETY DECISIONS SHOULD NOT BE MADE ON THE BASIS OF FEAR OF CERTIFICATE ACTION; BUT OFTEN THEY ARE. I'M NOT SURE HOW THIS COULD HAVE BEEN PREVENTED. PBI COULD HAVE BROKEN EITHER ACFT. OUT OF THE PATTERN; OR GIVEN US S-TURNS OR A 360 DEG; BUT THEY ARE TASKED WITH SEQUENCING ON PREDICTABLE; MIXED PERFORMANCE; FIXED AND ROTARY-WINGED TFC WHICH IS USUALLY MODERATE TO HVY VOLUME INTO 2 CLOSELY SPACED; PARALLEL RWYS! ALL THAT PLTS AND CTLRS CAN DO IS WATCH THE PARALLEL TFC CLOSELY; AND BE READY TO PERFORM EVASIVE MANEUVERS.