COMMUTER GIVEN EDC CANCELS IFR; DEPARTS VFR TO PICK UP IFR INFLT.

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ISSUED AN EDC OF XX55Z; GATE SCHEDULED DEP WAS XY24 CDT; BUT INBOUND FLT DELAYED; MAKING ACTUAL DEP XY32. CAPT WANTED TO TAXI OUT AND WAIT. HE THEN DECIDED TO DEPART VFR. I QUESTIONED IF THIS WAS LEGAL WITH CTL TWR OPERATING. HE INSISTED IT WAS AND INSTRUCTED ME TO NOTIFY GND OF INTENT TO DEPART VFR. DUE TO HVY TFC; WE DEPARTED ONLY 5 MINS (APPROX) BEFORE OUR EDC. GND HAD CANCELLED OUR CLRNC; SO I REFILED INFLT WITH MOBILE RADIO. HOUSTON CTR THEN DID NOT ACCEPT US AS IFR UNTIL NEARLY 50 MI OUT FROM MOB; WHICH IS THE COMPANY LIMIT TO BE VFR FROM DEP ARPT. WE DISCUSSED THE LEGALITY OF THIS ACTION; BUT ALL THE CAPT'S EXAMPLES STEMMED FROM NON-CTLED FIELDS. ZHU SEEMED IRRITATED AT US; AND QUESTIONED THE RATIONALE OF OUR ACTION. AFTER LNDG; I LOOKED UP SAID CASE IN OUR STANDARD PRACTICE AND THE ONLY EXCEPTION APPEARS TO THE IFR DEP IS FROM AN UNCTLED FIELD; AND ONLY IN VFR CONDITIONS; AND IF AN IFR CLRNC IS OBTAINED WITHIN 50 NM FROM DEP ARPT. REGARDLESS OF LEGALITY; THE DECISION TO CANCEL IFR; DEPART VFR WAS NOT PRUDENT; ESPECIALLY CONSIDERING THE FACT THAT IT ONLY SAVED APPROX 5 MINS; IF THAT. WHILE NOT ACTUALLY IN COMMAND; I WILL VOICE MY FEELINGS. CLRER WORDING; WHICH CAN'T BE INTERPRETED EITHER WAY; WOULD ALSO HELP.