DC9 CREW MISINTERPED CLRNC AND SLOWED TO FINAL APCH SPD EARLY.

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CAPT WAS PF. CLRED TO MAINTAIN 5000 FT AND 170 KTS TO ROBBI INTXN (13.9 DME) AND CLRED ILS RWY 21L APCH. AT ROBBI INTXN; ACFT CONFIGURED TO LAND AND SLOWED TO 133 KTS. AT APPROX 10 DME; DTW FINAL CTL MONITOR QUERIED US 'WHAT SPD ARE YOU DOING ANYWAY?' WE REPLIED '133 KTS.' HIS RESPONSE WAS 'YOU WERE TOLD 'TO MAINTAIN 170 KTS UNTIL THE MARKER.' WE REPLIED 'NO; WE WERE TOLD UNTIL ROBBI.' HE ASKED US TO SPD BACK UP UNTIL THE MARKER (4 NM). I REPLIED 'UNABLE.' I FELT MAKING A BIG CHANGE IN SPD AND/OR CONFIGN 4 MI FROM THE MARKER AND THEN CHANGING BACK WAS UNSAFE DUE TO THE 2000 FT RVR CONDITIONS. I WOULD HAVE PREFERRED A GAR. TRACKING DOWN THE GS 9 MI OUT IN THESE CONDITIONS IS NO PLACE TO MAKE BIG CHANGES. HE THEN INSTRUCTED US TO CALL HIM ON THE GND AND GAVE US THE PHONE NUMBER 'FOR A POSSIBLE PLTDEV.' I DON'T BELIEVE THIS WAS THE APPROPRIATE TIME TO BE TELLING US THIS. HE SHOULD HAVE WAITED UNTIL WE WERE ON THE GND. WHEN I CALLED HIM; HE TOLD ME WE WERE WRONG ABOUT THE CLRNC AND WE WERE SUPPOSED TO MAINTAIN 170 KTS TO THE MARKER. I TOLD HIM WHY I WAS UNABLE TO SPD UP FOR 4 MI AND THAT WE CERTAINLY READ BACK THE CLRNC AS WE UNDERSTOOD IT. INCIDENTALLY; FLYING THAT FAST TO THE FAF IS SIGNIFICANTLY DIFFERENT FROM NORMAL OPERATING PROCS.