A B757 ON A VISUAL APCH TO LAS RWY 25L EXPERIENCES A CONFLICT WHEN A B737 CROSSES UNDER FOR RWY 25R. THE B737 WAS MAINTAINING VISUAL SEPARATION FROM THE B757. TFC WAS NOT ISSUED TO THE B757 FLC.

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WE WERE CLRED TO INTERCEPT THE RWY 25L LOC AND PROCEED INBOUND TO RWY 25L AND GIVEN A DSCNT CLRNC TO 6500 FT. THE FO SAID HE SAW THE TFC ON THE TCASII; BUT IT WAS NO FACTOR (SHOWING WHITE). NEXT WE GET A TCASII RA (RED). I LOOKED OUT THE L WINDOW AND SAW THE ACFT; A B737 (ACR Y) COMING AT US ABOUT 10 O'CLOCK AND SLIGHTLY BELOW US. DID NOT HAVE TIME TO REACT TO TCASII AND HAD ACFT VISUALLY. I KNEW HE WOULD CLR BELOW US. AS HE WENT UNDER US THE TCASII INDICATED A 500 FT SEPARATION. I ASKED APCH WHAT HE WAS DOING THERE AND WHY THEY DID NOT TELL US ABOUT HIM. APCH SAID HE WAS SORRY FOR NOT ADVISING US AND THAT ACR Y SAID HE HAD US VISUALLY AND ACCEPTED A VISUAL BEHIND US. AT THIS TIME APCH CLRED US FOR A VISUAL TO RWY 25L AND CHANGE TO TWR FREQ. I FEEL THAT ACR Y FAILED TO MAINTAIN PROPER SPACING AFTER ACCEPTING THE VISUAL APCH. THE PROB SEEMS TO BE THAT TOO MANY OF US IN THE AIRLINE BUSINESS ARE ACCEPTING VISUAL APCHS AT BUSY ARPTS TO SAVE TIME AND MONEY AND THAT TOO MANY CTLRS ARE ASKING US -- OR AT TIMES GIVING US -- A VISUAL TO MAKE THEIR JOBS EASIER. I ALSO BELIEVE MOST CTLRS ARE OVERLOADED AT TIMES DUE TO THE SYS. ONE MAJOR PROB I FIND AT LAS VEGAS IS THAT CTLRS DO NOT ADVISE YOU OF TFC AS THEY SHOULD. THEY WILL CLR YOU FOR AN APCH AND NEVER TELL YOU THAT YOU ARE BEHIND A HVY OR WHAT YOUR SPD IS IN RELATION TO THE TFC. SUPPLEMENTAL INFO FROM ACN 391862: LAS APCH CLRED US FOR THE VISUAL APCH TO RWY 25L. PASSING THROUGH 8000 FT WE SAW ANOTHER ACR B737 PASS FROM L TO R DIRECTLY UNDER US. TCASII SHOWED A VERT SEPARATION OF 500 FT AND GAVE US AN RA TO CLB. WE STARTED TO CLB AND QUERIED THE CTLR WHO TOLD US THE OTHER CARRIER HAD US IN SIGHT AND WAS TO HAVE KEPT VISUAL SEPARATION.