ALTDEV ALT EXCURSION AFTER RECEIVING TCASII RA DURING APCH.

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WE WERE APPROX 15 MI E OF MONROE INBOUND FOR LNDG WHEN MLU APCH CTL CALLED OUT TFC IN OUR 12 O'CLOCK THAT WAS CLBING TO 4000 FT ASSIGNED. (WE BOTH WERE UNDER MLU APCH CTL.) WE WERE LEVEL AT 5000 FT AND INITIALLY OBSERVED THIS ACFT ON TCASII CLBING THROUGH 2800 FT. WE WATCHED AS HE CLBED THROUGH 4200 FT AT WHICH TIME WE RECEIVED A TCASII RA WHICH COMMANDED A 2000 FPM CLB. I CANNOT VERIFY SEPARATION EITHER VERTICALLY OR HORIZLY AS WE NEVER DID VISUALLY SEE THE APCHING TFC. WE FOLLOWED THE TCASII RA; CLBING TO 6000 FT; RPTED OUR DEV FROM ASSIGNED ALT AND THEN RETURNED TO 5000 FT WHEN THE TFC HAD PASSED BENEATH US. MONROE HAD ONLY A FEW ACFT TO CTL AT THIS TIME. I QUESTION THEIR DECISION TO RUN 2 ACFT HEAD-ON WITH ONLY 1000 FT SEPARATION. PERHAPS WE SHOULD HAVE ASKED FOR A VECTOR; BUT OUR ATTN WAS ON OUR APCH AND THE TSTMS THAT WERE COMPLICATING IT. TIME WAS A FACTOR AS WE HAD LITTLE WARNING. ONE COMMENT ABOUT OUR TCASII. IT WAS EQUIPPED WITH THE FLT LEVEL CIRCUITRY THAT ALLOWS DEPICTION OF ACTUAL TARGET ACFT ALT BELOW 18000 FT AS OPPOSED TO DEV FROM HOST ACFT ALT. I FIND THE FORMER EASIER TO INTERPOLATE QUICKLY. SUPPLEMENTAL INFO FROM ACN 247331: AFTER DEPARTING MLU I WAS TO MAINTAIN 4000 FT. PRIOR TO REACHING THIS ALT I WAS GIVEN A TA FOR TFC DSNDING FOR MLU OUT OF 5000 FT; AN MLG. AS I WAS CLBING THROUGH 3800 FT AND LOOKING FOR THE TFC; THE MLG WINGTIP LNDG LIGHTS AND NOSE SECTION CAME INTO VIEW OUT OF THE CLOUDS. THE ACFT APPEARED TO BE AT OR VERY NEAR MY ALT AND CLOSING RAPIDLY. I FELT THAT EVASIVE ACTION WOULD BE NECESSARY IN THE FORM OF A CLB SINCE I WAS ALREADY CLBING AND THE MLG APPEARED IN A DSCNT FOR THE ARPT. I CONTINUED CLBING TO 4200 FT AND COULD TELL BY THIS TIME THE MLG WOULD PASS ABOVE ME AND I COULD ALSO TELL THE CAPT OF THE MLG HAD ADDED PWR AND STARTED A PITCH UP OF THE NOSE OF THE ACFT TO BEGIN EVASIVE ACTION HIMSELF; SO I DROPPED THE NOSE OF MY ACFT UPON SEEING THIS AND WENT BACK TO MY ALT ASSIGNMENT. I KNEW I COULD AVOID A COLLISION WITH THE MLG BUT I DID NOT UNDERSTAND WHY 2 ACFT; 1 LEAVING THE ARPT AND 1 ARRIVING; HAD TO BE BROUGHT SO CLOSE HORIZLY AND THEN TO HAVE ONLY 1000 FT VERT SEPARATION BTWN BOTH IN MARGINAL CONDITIONS WITH TSTMS IN THE IMMEDIATE AREA TO THE NE. I FEEL THAT THIS SEPARATION CRITERIA IS NOT SUFFICIENT TO PROVIDE MAX SAFETY DURING ACFT OPS; ESPECIALLY IN REDUCED VISIBILITY. IF THE VERT SEPARATION CAN'T BE WIDENED; THEN A CTLR WITH VECTORING INSTRUCTIONS COULD HAVE MOVED THESE ACFT AWAY FROM EACH OTHER.