B757 ARR TO SFO WITH NCT EXPERIENCED TCASII RA WITH ISSUED VFR TFC THAT WAS MAINTAINING VISUAL SEPARATION WITH THE RPTING ACFT.

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ATC GAVE US A SPD RESTR OF 290 KTS; KEPT US HIGH AND THEN GAVE CEDES AT 11000 FT AT 250 KTS. ASKED FOR RELIEF FROM SPD AND THEY COULDN'T ACCOMMODATE DUE SEQUENCING INTO SFO. FO FLYING -- WAS HIGH; BUT WAS GOING TO MAKE RESTR WITH SPD BRAKES FULLY DEPLOYED. ATC ADVISED US TO LEVELOFF AT CEDES AT 12000 FT NOW AS HE HAD VFR TFC AT 11500 FT. WE ACKNOWLEDGED. TFC CAME INTO TCASII DISPLAY AND SHOWED 11700 FT VERSUS 11500 FT. ATC QUERIED GA AIRPLANE AND HE SAID HE WAS AT 11500 FT AND HAD US IN SIGHT. WE NEVER HEARD ANY OF THIS CONVERSATION AS I THINK HE WAS ON ANOTHER FREQ. WE WERE A BIT UNCOMFORTABLE AS WE WATCHED TCASII DISPLAY AND WERE DISCUSSING IT AMONGST CREW WHEN ATC SENT US TO NORCAL APCH. ATC SAID; 'VERIFY YOU HAVE THE GA TFC IN SIGHT;' TO WHICH I REPLIED 'NEGATIVE;' AND AS I WAS ABOUT TO TELL HIM WE WANTED A TURN OR A VECTOR HE GAVE US A COLLISION OR TA (NOT SURE WHICH) TO CLB TO 13000 FT. AT THIS POINT WE HAD OBSERVED THE BARON ACFT ON TCASII TO BE AT 11800 FT. SIMULTANEOUSLY WE RECEIVED A TCASII CLB RA WITHOUT ANY TFC ALERT. WE RESPONDED ALTHOUGH SOMEWHAT LATE AS FO WAS SLOW IN RESPONDING DUE TO FATIGUE AND A VERY SHORT LAYOVER WITH POOR SLEEPING ACCOMMODATIONS AT MIA. WE PERFORMED THE RA AND ADVISED ATC THAT THAT WAS UNACCEPTABLE TO HAVE HAPPENED AS SUCH. I LATER SPOKE ON PHONE TO OPS MGR AT ZOA AND ADVISED HIM OF MY INTENT TO FILE AN NMAC. HE UNDERSTOOD. I INFORMED HIM OF WHAT I FELT TO BE AN ILLOGICAL AND POOR TECHNIQUE OR PROC ON ATC'S PART TO WHICH HE REPLIED THAT THAT PROC IS THE ATC HANDBOOK WAY TO DO IT. SPECIFICALLY I QUESTION THE LOGIC OF RELYING ON A GA ACFT TO MAINTAIN VISUAL SEPARATION WITH AN ACR OR HIGH PERFORMANCE ACFT OP WHEN THE LATTER DOES NOT SEE THE FORMER. WHILE IT IS LEGAL IT SEEMS BACKWARDS AT BEST; IN THAT THE ACR/CPR PLANE SHOULD BE THE ONE TO RELY ON VERSUS SOMEONE WHO JUST MAY BE A LEISURE FLYER. AS A GA PLT I HAVE NO PROB WITH THEM; IN GENERAL; HOWEVER; IN THE MIX OF AIRLINERS THE PROC SHOULD REQUIRE THE 'PROFESSIONAL CREW' TO ACQUIRE THE TFC AND IF UNABLE TO THEN MANDATE A VECTOR. THIS IS THE SECOND TIME IN ABOUT 6 MONTHS THAT I HAVE HAD A TFC ENCOUNTER ON THE CEDES ARR WITH A VFR ACFT REQUIRING AN RA MANEUVER. THE FIRST TIME I BELIEVED IT TO BE AN ERROR; HOWEVER NOW I'M BEGINNING TO BELIEVE THIS IS COMMONPLACE. IT MIGHT BE LEGAL; BUT IT IS CERTAINLY FRAUGHT WITH DANGER; AND IS AN ACCIDENT WAITING TO HAPPEN.