FLC TOOK EVASIVE ACTION ACCOUNT TCASII RA. TFC WAS IN SIGHT.

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DURING DSCNT INTO DFW ARPT; ON THE SCURRY 1 ARR; WE WERE INSTRUCTED TO CROSS PORKS INTXN AT 11000 FT. A (NE-SW) LINE OF SMALL TSTMS WAS BTWN SCY AND DFW. TFC WAS STILL ABLE TO GET INTO DFW WITH MINOR DEVS; BUT TFC IN THE AREA WAS GREATER THAN NORMAL DUE TO THE COMPRESSED AIRSPACE CAUSED BY INVASION OF AIRSPACE BY TSTM ACTIVITY. WE LEVELED AT 11000 FT PRIOR TO REACHING SCY VOR. THE APCH CTLR ISSUED US AND ANOTHER FLT TFC INFO OF VFR TFC. WE INITIALLY DID NOT SEE THE TFC. ATC SPECIFIED THE TFC'S TRANSPONDER (MODE C) INDICATED IT WAS NEAR OUR ALT. OUR TCASII WAS OPERATIVE; AND SATURATED WITH TFC DISPLAYS AHEAD. MOST OF THE TFC WAS OPPOSITE DIRECTION; OUTBOUND DFW TFC; AT 10000 FT; OR 1000 FT BELOW US. MY CREW SIGHTED TFC AT OUR 11 O'CLOCK POS. IT WAS IDENTED AS A LIGHT AIRPLANE. SIMULTANEOUSLY; ATC ISSUED A SHARP TURN TO THE L. THIS ACTION ALONE; WOULD HAVE ALLOWED PLENTY OF TFC SEPARATION BTWN US AND THE TARGET TO MAINTAIN SAFE SEPARATION. THE TCASII THEN ISSUED A 'DSND' COMMAND. THE PF INITIATED AN AGGRESSIVE 'DIVE-INPUT'. WE EXCEEDED 300 FT OFF OUR ASSIGNED ALT OF 11000. I WAS IN THE R SEAT; HAD THE SUBJECT ACFT IN SIGHT DURING THESE MOMENTS. THE ACFT WAS CLOSE ENOUGH TO IDENT IF AS A LIGHT ACFT; BUT STILL TOO FAR AWAY TO ASCERTAIN TYPE; OR EVEN IF IT WAS A SEL OR TWIN ACFT TYPE. THEREFORE; WE DIDN'T COME CLOSE TO A NEAR MISS OR JEOPARDIZED SAFETY. SINCE WE WERE ABEAM THE ACFT BEFORE THE EVASIVE DIVE ACTUALLY BEGAN; WE GAINED NOTHING WITH THE TCASII DIVE. POLICY MANDATES WE FOLLOW TCASII COMMANDS; EVEN IF WE HAVE SUBJECT INTRUDER ACFT IN SIGHT. INASMUCH AS WE HAD THE INTRUDER IN SIGHT; HORIZ SEPARATION WAS VISUALLY ASSURED; IT TROUBLED ME TO LEAVE 11000 FT; TAKE A DIVE TOWARDS A DEP CORRIDOR WITH CONSIDERABLE DFW OUTBOUND TFC AT 10000 FT. ATC DID ISSUE AN 'OFF CORRIDOR' TURN DSCNT (TO THE S); BUT ONLY AFTER OUR ACTION WAS INITIATED. THE SO-CALLED INVADER ACFT COULD VERY WELL HAVE BEEN PERFECTLY LEGAL. I DON'T KNOW. HE WAS OUTSIDE THE TCA; ATA'S; OTHER POSITIVE CTLED AIRSPACE. IF THE ACFT WAS CLBING/DSNDING; 11000 FT COULD HAVE BEEN A COINCIDENTAL; BUT LEGAL; ALT. (LIGHT ACFT CLB VERY SLOWLY AT THOSE ALTS.) IF IT WAS IN CRUISE; I SUSPECT A WRONG ALTIMETER SETTING OR ELSE POOR ALT CTL DISCIPLINES. 11500 FT WOULD HAVE BEEN PROPER CRUISE ALT FOR HIS DIRECTION OF FLT. HE WAS VERY CLOSE TO 11000. SINCE ATC ISSUED A DSCNT CLRNC DURING EARLY PORTIONS OF OUR DIVE; WE MERELY KEPT DSNDING AND DID NOT RETURN TO 11000 FT. THE ATC FREQ WAS EXTREMELY BUSY WITH TFC COMPRESSION; TFC SATURATION; NUMEROUS RTE DEVS; DUE TO TSTM ACTIVITY. IF WE HAD BEEN IN ACTUAL IMC CONDITIONS; OR IF TFC WAS NOT IN SIGHT; I FEEL THE ESCAPE (RA) MANEUVER WOULD HAVE BEEN PROPER. HOWEVER; WE CLRLY HAD TFC IN SIGHT WITH POSITIVE HORIZ SEPARATION ASSURED. THEREFORE; I FEEL THIS TCASII COMMAND WAS SAFE AND CONSERVATIVE; BUT NOT NECESSARY (I.E.; 'OVERKILL'; BUT OK AS LONG AS IT'S IN THE CONSERVATIVE DIRECTION).