TCASII GAVE FAULTY INFO IN A SEE AND BE SEEN SIT.

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DEP CTL CALLED VFR TFC 12 O'CLOCK; 8 MI; INDICATING 14400 FT 'DO YOU HAVE HIM IN SIGHT.' TCASII WARNED 'TFC; TFC' AND ALL 5 PAIRS OF EYES (3 PLTS; 1 FAA ATC JUMPSEATER; 1 DEADHEAD PLT JUMPSEATER) WENT OUTSIDE. APPROX 30 SECONDS LATER (AT 13400 FT MSL); TCASII GAVE A MONITOR VERT SPD' AND FO AND SO SPOTTED TFC AT 12 - 12: 30 POS; TWIN (BEECH?) L WING HIGH (R TURN) VERY SLIGHTLY ABOVE US (800 FT). CAPT INITIATED DSCNT AND L TURN AND TCASII TOLD US TO 'CLB; CLB.' WE PASSED ABOUT 700 FT BELOW AND 800þ1000 FT HORIZ TO TFC; DSNDING TO 13000 FT AND TURNING 25-30 DEGS OFF DIRECT COURSE. TCASII GAVE BAD RESOLUTION; BUT MAYBE DEP COULD HAVE GIVEN US SOME AVOIDANCE TURN A BIT SOONER. ALL OF US WERE OPERATING LEGALLY; AND HAD WE LEVELED OFF AT 14000 FT WE WOULD HAVE MISSED BY 400 FT OR SO (PROVIDING THE OTHER ACFT WASN'T DSNDING; BUT WHY RISK IT?) SEE AND BE SEEN IN A BUSY DEP CORRIDOR DOESN'T ALWAYS WORK! I SUGGEST FURTHER EMPHASIS TO VFR PLTS TO REQUEST FLT FOLLOWING; PARTICULARLY WHEN TRANSITING CERTAIN BUSY DEP RTES AND ESPECIALLY WHEN VFR AROUND MAJOR HUB ARPTS. WHILE BOTH ACFT WERE OPERATING LEGALLY; THE LIKELIHOOD OF AN INFLT COLLISION WAS VERY HIGH IN THIS CASE -- AND REALLY THE CAPT USED OUTSTANDING COCKPIT RESOURCE MGMNT SKILLS BY DSNDING AND TURNING BASED ON INPUT FROM FO AND SO; AS THE CTR WINDSHIELD POST BLOCKED HIS VIEW OF THE OTHER ACFT INITIALLY.