EMB120 ACFT ON STAR ARR HAD TCASII TA; BUT TOOK EVASIVE ACTION. ATC HAD RPTED TFC AND FLC EVENTUALLY SAW IT AS IT PASSED OFF THE WING AND BEHIND THEM. RPTR FO SAYS THAT THE DESIGN OF THE AIRSPACE IN THAT AREA CAUSES TCASII ALERTS REGULARLY WITH UNCTLED VFR ACFT.

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ON DSCNT INTO DFW; ZFW CLRED US TO 'FLY THE BONHAM 1 ARR; CROSS BONHAM AT AND MAINTAIN 7000 FT.' WE WERE TOLD TO 'CONTACT APCH' JUST N OF THE BYP VOR. WE CHKED IN WITH APCH AS WE WERE LEVELING OFF AT 7000 FT OVER THE VOR. THE CTLR ACKNOWLEDGED OUR CHK-IN AND ISSUED 'TFC AT 1-2 O'CLOCK; 2 1/2 MI; VFR AT 7500 FT UNVERIFIED.' WE REFED THE TCASII; WHICH SHOWED THE TFC AT 2 O'CLOCK; 1 1/2 MI; +400 FT (SOLID DIAMOND). SINCE WE DID NOT HAVE THE TFC IN SIGHT; THE CAPT DISENGAGED THE AUTOPLT; AND TOOK EVASIVE ACTION TO THE L. AT THE SAME TIME WE RECEIVED A TA FROM THE TCASII; WHICH SHOWED THE TFC 2-3 O'CLOCK INSIDE OF 1 MI RANGE. IN OUR L TURN; I SPOTTED THE TFC PASSING OFF OUR R WING AND BEHIND US; ABOUT 1/4 MI AWAY AND NO MORE THAN 100 FT OR SO ABOVE OUR ALT. WE ADVISED APCH OF OUR ACTIONS; AND RETURNED TO THE ARR. ONE OF THE MAIN CONTRIBUTING FACTORS TO THIS INCIDENT IS THE CURRENT POLICY AT DFW OF BRINGING ARRIVING HIGH PERFORMANCE TURBOPROPS IN THROUGH THE SIDE OF THE CLASS B AIRSPACE; RATHER THAN IN THROUGH THE TOP; LIKE THE JETS. THIS; COMBINED WITH THE FACT THAT THE 'CORNERPOST' VORS HAVE BEEN MOVED ABOUT 20 MI FURTHER OUT; PUTS US RIGHT IN THE MIDDLE OF THE 6000-8000 FT AVERAGE CRUISING ALTS FOR THE MAJORITY OF LIGHT ACFT. THERE IS A STRETCH OF AIRSPACE ABOUT 25 MI LONG BTWN THE CORNERPOSTS AND THE EDGE OF CLASS B WHERE WE ARE REQUIRED TO BE AT THESE ALTS; PUTTING THE SAFETY OF OUR CREWS; ACFT; AND ESPECIALLY OUR PAX IN JEOPARDY. UNFORTUNATELY; TFC CONFLICTS OF THIS KIND ARE NOT UNCOMMON. ON A RELATED NOTE; ONE FACTOR THAT MIGHT HAVE PLAYED A PART IN THIS INCIDENT IS THE TCASII DISPLAY ITSELF. ON OUR E120'S THE TCASII DISPLAY IS A SINGLE SCREEN; CENTRALLY LOCATED ON THE FORWARD PEDESTAL BTWN THE PLTS. WHILE THIS LOCATION MAKES IT POSSIBLE FOR BOTH PLTS TO GET TCASII INFO FROM A SINGLE DISPLAY; IT IS NOT ANYWHERE NEAR THE PLT'S NORMAL INST SCAN. ALSO; IN ITS CURRENT LOCATION; IT IS FREQUENTLY IN DIRECT SUNLIGHT; MAKING THE DISPLAY UNREADABLE UNTIL ONE OF THE PLTS SHIELDS IT WITH HIS HAND. THIS ALL TAKES PRECIOUS SECONDS AWAY FROM ACQUIRING THE TFC; AS THE PLTS MUST FIRST LEAVE THEIR REGULAR SCAN; TURN THEIR HEADS; LOOK DOWN; SHIELD THE DISPLAY IF NECESSARY; DETERMINE THE THREAT; THEN ATTEMPT TO ACQUIRE THE TFC VISUALLY. THIS WAS THE SECOND NEAR MISS IN AS MANY DAYS FOR US -- BOTH OF WHICH WERE MADE INTO NON EVENTS (CONSIDERING THE ALTERNATIVE) BY HAVING TCASII ON BOARD. HOWEVER; A TCASII UNIT THAT WAS INTEGRATED INTO THE VSI (AS IT IS ON MANY ACFT) AND THEREFORE PART OF THE PLT'S NORMAL SCAN; MIGHT HAVE ALLOWED US TO REALIZE THE THREATS A LITTLE SOONER; GIVING US MORE TIME TO MANEUVER CLR. I HAVE BEEN FLYING WITH TCASII FOR 3 1/2 YRS; AND HAVE FOUND IT TO BE ALMOST ESSENTIAL AND CERTAINLY USEFUL IN THE COCKPIT. I'M GLAD WE HAVE THIS EQUIP ON OUR ACFT.