FLC OF S80 HAS TCASII RA TO DSND WHEN CLOSE IN ON FINAL OF VISUAL APCH. THEY HAVE TFC IN SIGHT AND DO NOT RESPOND TO THE TCASII RA.

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ATC FACILITY: DFW TWR. LOCATION: 110.3/ON LOC AND GS BEAM FOR RWY 17C DFW 2-3 DME. DURING SIMULTANEOUS VISUAL APCHS TO RWY 17L/17C AT DFW WE WERE VECTORED ABOVE AND BEHIND TFC TO RWY 17L TO INTERCEPT FINAL FOR A VISUAL APCH TO RWY 17C. WE WERE ABLE TO OBSERVE THE OTHER ACFT AND VERIFY ITS TCASII SYMBOL THROUGHOUT THIS EVENT. ESTABLISHED ON FINAL WITH VISIBILITY UNRESTR WE COULD MONITOR OUR RELATIVE POS TO THE OTHER ACFT. THE COMMENT WAS MADE THAT THE OTHER ACFT APPEARED CLOSER THAN IT SHOULD BE. ITS SYMBOL ON THE TCASII DISPLAY WAS TOO CLOSE TO OURS TO JUDGE DISTANCE. IT MAINTAINED A POS 200-300 FT ABOVE US AT 2-3 MI WITHOUT A 'TFC' CALL; WE GOT A 'DSND' COMMAND FROM TCASII WITH A VSI RED/GREEN COMMAND OF 1500- 2000 FPM. AFTER A HASTY SURVEY OF OUR CIRCUMSTANCES AND CONTINUED CLOSE MONITORING OF OUR BOOGEY AT 9 O'CLOCK POS WE INSURED OUR OWN SEPARATION AND CONTINUED THE APCH TO A LNDG. EVEN THOUGH IDEAL CIRCUMSTANCES FOR PARALLEL APCHS EXISTED; A TCASII COMMAND OCCURRED THAT I FEEL WAS GROSSLY INAPPROPRIATE AND DANGEROUS. WE HAD BEEN VISUALLY MONITORING THE CONFLICTING TFC SINCE DOWNWIND. HAD WE BEEN IMC; WE WOULD HAVE BEEN PRESENTED WITH AN UNRESOLVABLE RA.