WHEN TURNING FINAL TO SJC RWY 30; AN MD80 INITIATES TCASII MANEUVER DUE TO POTENTIALLY CONFLICTING VFR C172 TRANSITIONING OPPOSITE DIRECTION FROM SJC.

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PROB: TCASII ADVISORY ON OTHER (AIRLINE ACFT) CAUSED CLB WHEN NOT NEEDED. CONTRIBUTING FACTOR WAS IMPRECISE ATC INSTRUCTION TO MY ACFT. DUAL TRAINING FLT DEPARTING SJC RWY 29 ON L DOWNWIND. APCH DIRECTED MY ACFT TO A POINT S OF SJC; THEN GAVE US 'RESUME OWN NAV; REMAIN CLR OF RWY 30 FINAL APCH COURSE' OR SIMILAR INSTRUCTION; NO DISTANCE SPECIFIED. DEST WAS SOUTH COUNTY (Q99) WHICH IS RIGHT UNDER THE FINAL FOR RWY 30. I DIALED IN THE RWY 30 LOC FREQ WHICH SHOWED FULL DEFLECTION. MY STUDENT SLOWLY DRIFTED FROM HEADING APPROX 120 DEGS TO ABOUT 110 DEGS MAGNETIC. ATC POINTED OUT TFC AS A JET TURNING TO THE FINAL FOR RWY 30. WE MADE VISUAL CONTACT AND WERE WELL CLR. THE JET RPTED A TA AND THEN RA. ATC THEN ASKED US TO STAY '2 MI' AND THEN 3 MI W OF THE FINAL. ATC'S INSTRUCTION TO 'REMAIN CLR OF THE APCH COURSE' WAS NOT SPECIFIC ENOUGH. WHAT WAS FINE FOR VFR SEPARATION OBVIOUSLY WAS NOT GOOD ENOUGH FOR TCASII. ATC NEEDS TO BE MORE SPECIFIC HERE AND SINCE DISTANCE IS HARD TO ESTIMATE PRECISELY TO AN INVISIBLE LINE (THE LOC); A DISTANCE MAY NOT BE ENOUGH. A VECTOR WORKS BETTER. ALSO; TCASII SEEMS TO CREATE PROBS WHEN NONE EXIST. THE JET HAD US IN SIGHT AND WE THEM. NEAREST DISTANCE WAS PROBABLY 1.5 MI HORIZ AND 500-1500 FT VERT; YET THE JET HAD TO CLB; LEADING TO A LESS STABLE DSCNT FOR HIM; AND A WIDE DIVERT AROUND THE LOC FOR US. DOES TCASII NEED TO ENFORCE IFR SEPARATION UNDER VFR CONDITIONS? PERHAPS IT'S A LIMITATION OF THE TECHNOLOGY; BUT IT DOES HAVE SAFETY (UNNEEDED GAR) AND EFFICIENCY IMPACTS ON OPS.