ACR CREW OBJECTS TO TOO CLOSE SPACING FOR TKOF BEHIND A B757.

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WE WERE NUMBER 2 FOR TKOF BEHIND A B757. I HAD JUST BRIEFED MY COPLT THAT MY CRITERIA FOR TAKING OFF BEHIND A 'HEAVY' ACFT WAS 1 MINUTE AND 45 SECONDS; BECAUSE I KNEW ATL'S POLICY WAS TO CLEAR US FOR TKOF SO THAT 'BY THE TIME YOU'RE OFF THE GND; YOU'LL HAVE 4 MI SPACING.' AND INDEED; AT ONE MINUTE AFTER THEY CLRED THE B757 FOR TKOF; WE WERE CLRED FOR TKOF. I TOLD THE TWR WE NEEDED ANOTHER 30 SECONDS; TO WHICH HE REPLIED; 'CANCEL TKOF CLRNC; POS AND HOLD.' (AT THAT TIME THE B757 WAS ABOUT 2 1/4 MI AHEAD OF US ON TCAS.) HE CROSSED AN ACFT DOWNFIELD AND THEN CLRED US FOR TKOF AGAIN. AS WE WERE ROLLING DOWN THE RWY; 20 KTS PRIOR TO V1; THE TWR CLRED THE ACFT BEHIND US FOR TKOF. IF WE HAD TO ABORT (WHICH WAS CLRLY POSSIBLE 20 KTS PRIOR TO V1) AND IF EITHER THE TWR; OR THE OTHER ACFT HAD RADIO PROBS; OR HE ALSO HAD TO ABORT; THINGS COULD HAVE BEEN VERY BAD. I KNOW ATL GETS BUSY; BUT THIS IS NOT THE RIGHT WAY TO APPROACH THE PROB! ON THIS DAY; AT THAT TIME; THERE WERE ONLY 2...POSSIBLY 3 ACFT BEHIND US. WHAT'S THE RUSH? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATED THAT HE DID NOT FEEL THE CTLR WAS TRYING TO PENALIZE HIM FOR TAKING THE EXTRA DELAY; BUT THAT THE TONE OF THE CTLR WAS SOMEWHAT CONDESCENDING. HE SEES NO NEED DURING LIGHT TFC TIMES TO RUN RIGHT UP TO THE LIMIT OF SPACING RULES AND FEELS THAT LESS EXPERIENCED CREWS MAY BE LED INTO A WAKE TURB SIT THAT IS HAZARDOUS. WHILE TAXIING IN ON A SUBSEQUENT ARR THE SAME DAY AT ATL; HE OBSERVED A SIMILAR INCIDENT THAT HE CONSIDERED QUESTIONABLE.