CLOSE PROX ACR-LGT ACR-SMT. LGT WAS TAXIING TO TERMINAL AND SMT MAKING AN ILS APCH TO RWY 21 AT DTW.

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ACFT HAD JUST LANDED ON RWY 21R AT DTW. QUARTERING TAILWIND SITUATION WAS NOTED DURING APCH. WIND WAS 090/8 KTS. APCH CTL SUPVR HAD CHANGED RWY CONFIGN FROM PARALLEL 3R&L APCHS TO 21R&L DUE TO 3R APCH LIGHTS OUT AND RVR DROPPING RAPIDLY. WE TAXIED CLR OF RWY 21R ON RWY 27 AND RPTED CLR OF THE RWY. TWR SWITCHED US TO GND AND WE WERE CLRED TO GATE VIA TXWY G. AFTER APPROX 30 SEC OF TAXI TIME APPROX ABEAM C CONCOURSE; I OBSERVED A LIGHT TWIN IN APPROX 20-25 DEG RIGHT BANK COMING AT US AT ABOUT AN ALT OF 75'. HE WAS WELL TO THE RIGHT OF US; XING FROM OUR RIGHT TO LEFT. THE PLT APPARENTLY SAW OUR ACFT AND PULLED HIS NOSE UP TO MISS US. I WOULD ESTIMATE THAT HE WAS 350-400' LEFT OF CENTERLINE WHEN I FIRST SAW HIM. THE N# WAS CLEARLY VISIBLE. WE NOTIFIED GND CTL OF THE SITUATION AS THE ACFT PASSED BEHIND US; AND HE WAS RATHER SPEECHLESS ABOUT THE WHOLE INCIDENT. I DISCUSSED THE SITUATION WITH THE TWR SUPVR ON THE PHONE AFTER SECURING OUR ACFT. LATER SEVERAL QUESTIONS CAME TO MIND. IF APCH WAS RADAR MONITORED; HOW DID THIS ACFT GET THIS FAR OFF CENTERLINE AND ALMOST COLLIDE WITH A TERMINAL AND TAXIING ACFT W/O NOTIFICATION? PROBABLY TOO LOW FOR RADAR CONTACT. PROFICIENCY OF THIS PARTICULAR PLT. THIS PLT OBVIOUSLY NEEDS ADDITIONAL INSTRUMENT TRNING--IS THIS WHAT WE CAN EXPECT FROM PART 93 INSTRUMENT PLTS IN THE FUTURE? IF SO; WE NEED A LOT MORE FIRE TRUCKS.