FLC OF MLG GIVEN VFR TFC 500 FT ABOVE PASSING IN CLOSE PROX. TCASII GAVE RA AND FLC DSNDED.

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OUR FLT SJC-SNA; WAS ON RADAR DOWNWIND; BEING VECTORED FOR ILS 19R APCH. WE WERE AT 3000 MSL 210 KCAS. APCH CALLED CONVERGING TFC (LIGHT FIXED WING) AT OUR 2 O'CLOCK POS 3500 FT MSL. TFC WAS VFR WITH APCH CTL LETTING HIM PASS THROUGH THE ARSA. TCAS SHOWED A 'PROXIMATE TARGET' AT THE POS AND ALT CALLED BY ATC. AS WE GOT CLOSER; TCAS ANNOUNCED 'MONITOR VERT SPD' ONE TIME FOLLOWED BY 'DSND; DSND' REPEATED TWICE. WE DSNDED TO APPROX 2500 FT MSL WHEN TCAS ANNOUNCED 'CLR OF TFC.' WE ANNOUNCED OUR REACTION TO THE RA TO COAST APCH. COAST APCH SEEMED VERY SURPRISED THAT AN 'RA' OCCURRED WHEN THERE WAS 'LEGAL SEPARATION.' AS LONG AS THE TCAS SOFTWARE IS PROGRAMMED TO KEEP A 1000 FT VERT SAFE DISTANCE BTWN ALT ENCODED ACFT THIS EVENT WILL OCCUR OVER AND OVER BTWN VFR AND IFR ACFT IN AN APCH DEP SITUATION. ATC NEEDS TO UNDERSTAND THAT JUST BECAUSE IT'S 'LEGAL SEPARATION' IN THEIR BOOK; DOESN'T MEAN TCAS WON'T REACT. QUESTION: BY OUR REACTION (DSNDING) TO THIS RESOLUTION ADVISORY; DID WE NOT ENDANGER MY ACFT AND PAXS AND MAYBE SOME OTHER ACFT BELOW US? TCAS IS GREAT; DON'T GET ME WRONG; BUT IF THE FAA IS GOING TO MANDATE ALL SCHEDULE AIR CARRIERS TO HAVE IT ON THEIR ACFT; ATC HAS TO KNOW HOW TO HANDLE TCAS EQUIPPED ACFT. DON'T MIX VFR AND IFR IN CLOSE PROX!