MLG MAKES APCH TO WRONG RWY. SIDE STEPS BUT VERY UNSTABLE APCH.

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ON ARR TO DFW; AFTER EARLY XA15 AM 'GET UP' AND ALMOST 7 HRS OF FLYING TIME IN 3 LEGS. VECTORED TO VISUAL APCH TO RWY 17L (OR SO WE THOUGHT) LOTS GOING ON. APCH CTL ASKED US FOR A CLOUD 'BASE RPT;' CALL THE FIELD; LOOK FOR TFC LNDG ON A W SIDE RWY; AND SWITCH TO TWR FREQ (11 NM OUT). FO FLYING 'CHATTERING' A MI A MIN AND MAKING NO ATTEMPT TO 'SANDWICH' HIS REMARKS BTWN RADIO CALLS FROM THE CTLR OR TWR. SWITCHED TO TWR FREQ AND WAS CLRED TO LAND. I THOUGHT HE SAID ON RWY 17L; THE USUAL RWY. AN ACFT WAS HOLDING SHORT OF RWY 17R; AND I ASSUMED HE WOULD BE CLRED INTO POS SHORTLY. RWY 17L WAS COMPLETELY CLR AND NO MENTION WAS EVER MADE ABOUT IT POSSIBLY BEING 'CLOSED.' I'M STILL NOT CERTAIN IF IT WAS OR NOT. AT ABOUT 800 FT AGL TWR ASKED IF WE WERE LINED UP ON RWY 17L. I REPLIED THAT WE WERE. TWR THEN STATED (TO OUR GREAT SURPRISE) THAT WE WERE CLRED TO LAND ON RWY 17R! I DO REMEMBER EARLIER ACKNOWLEDGING OUR CLRNC TO LAND; BUT I DO NOT RECALL IF I STATED THE RWY (I USUALLY AM VERY GOOD ABOUT THIS); NOR DO I RECALL TWR SAYING CLRED TO LAND ON RWY 17R. IN FACT; I AM REASONABLY CERTAIN THAT APCH CTL HAD CLRED US FOR A VISUAL TO THE L; AS THAT IS WHAT WE HAD BRIEFED AND HAD THE ILS ('BACKUP') TUNED TO. IN ANY CASE; TWR SHOULD HAVE AT LEAST PUT SOME FORM OF EMPHASIS ON THE WORD 'R;' IF HE IN FACT CLRED US SO. IT IS VERY UNUSUAL AT DFW TO BE CLRED TO LAND ON RWY 17R WHEN ACFT ARE WAITING TO USE IT FOR DEP. I ASKED TWR 'DO YOU WANT US TO LAND ON THE R?' HE SAID YES; BUT OFFERED NO REASON. AT THIS POINT THE FO STATED THAT 'HE COULD STILL DO IT.' TO ME IT LOOKED 'IFFY' BUT I WAS WILLING TO AT LEAST LET HIM TAKE A SHOT AT IT. THIS WAS; IN HINDSIGHT; A BAD DECISION. THE REMAINING APCH WAS NOT NECESSARILY UNSAFE (SOMEWHAT AKIN TO THE 'RIVER VISUAL' AT DCA); BUT I DID FEEL A BIT 'UNCOMFORTABLE;' AS; I AM SURE; DID OUR PAX. TECHNICALLY A 'GAR' WAS CALLED FOR BY COMPANY. PROC; DUE TO 'UNSTABLE APCH BELOW 500 FT AGL;' PRIMARILY DUE TO BANK ANGLES NEEDED TO SIDE STEP ON SUCH SHORT NOTICE. WE TOUCHED DOWN WITH ABOUT 9500 FT REMAINING AND THE LNDG WAS 'AVERAGE' IN QUALITY. IN THE FUTURE; I WILL BRIEF MY FO'S TO ATTEMPT TO 'SANDWICH' THEIR COMMENTS AND CONFIGN REQUESTS BTWN RADIO CALLS IF AT ALL POSSIBLE; AND I WILL DIRECT A GAR IF SUCH AN EVENT SHOULD EVER OCCUR AGAIN.