ACR AT CRUISE RECEIVES TCASII TA THEN RA. RESPONDS WITH DSCNT.

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WITH NO WARNING FROM ATC. WE RECEIVED A TA FROM OUR ONBOARD TCASII; SHORTLY FOLLOWED BY A DSND COMMAND. WE SIMULTANEOUSLY INFORMED CTR WE WERE DSNDING FROM OUR ASSIGNED ALT OF FL240; AND DEPARTED FOR A LOWER ALT. THE CONFLICT WAS RESOLVED AS WE REACHED FL235. AFTER CLBING BACK TO FL240; ATC INFORMED US THAT THE CONFLICTING TFC WAS A CORP JET DSNDING AT A RATE OF 3000 FPM FROM FL290 TO FL250 ON A HEAD-ON DIRECTION. THE CTLR SAID THE CORP JET WAS TO PASS 3 MI W OF OUR POS. DUE TO THE 1000 FT AND 3 MI SEPARATION; THE CTLR FELT IT UNNECESSARY TO ADVISE US OF THE MERGING TFC (JET WAS TOLD TO EXPEDITE HIS DSCNT). CTLRS NEED TO INCORPORATE INTO THEIR PROCS ACTIONS WHICH WILL NOT SET OFF TCASII PROBLEMS. IF ATC DSNDS ACFT WITH HEAD ON COURSES AT EXPEDITIOUS RATES; AS WAS THE CASE HERE; THEY'RE GOING TO HAVE ACFT BREAKING OUT OF THE PATTERN ALL OVER THE PLACE. RECOMMEND 30 DEG TURNS AWAY FROM OTHER TFC FOR 'EXPEDITING' CLBS OR DSCNTS WHEN NEWLY ASSIGNED ALTS ARE WITHIN 1000 FT OF OTHER ACFT. SUPPLEMENTAL INFO FROM ACN 206178: THE CTR CTLR TOLD US TO DISREGARD THE TCASII AND MAINTAIN OUR ALT. WE TOLD THE CTLR WE COULDN'T DO THAT. HE THEN TOLD US NOT TO DSND BELOW FL220 AND TFC WAS NOT A FACTOR. WE WERE CONCERNED ABOUT THE FACT OF A CTLR TELLING US TO IGNORE A TCASII RA ESPECIALLY WHEN WE WERE IMC AND COULD NOT SEE THE CONFLICTING ACFT.