ACR X GIVEN S TURN ON SHORT FINAL FOR INTERSECTING RWY DEP.

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ACR X AFTER RPTING THE ARPT IN SIGHT AT 25 MI; WE WERE CLRED FOR THE VISUAL (TIPTOE) TO RWY 28L; AND A SPD OF 210 KTS. WE WERE THEN INFORMED THAT WE WERE FOLLOWING A JETSTREAM; AND TO SLOW TO 180 KTS. APCH TOLD US THAT WE HAD 'TFC AT 2 O'CLOCK AND 6000 FT; VISUAL FOR THE R. RPT HIM IN SIGHT.' I REPLIED THAT WE HAD AN AIRPLANE THAT COULD BE THE TFC. APCH SAID; 'MAINTAIN VISUAL SEPARATION ON THE TFC. IF HE FALLS BEHIND YOU; HE WILL MAINTAIN VISUAL SEPARATION ON YOU.' WE WERE SWITCHED TO TWR AT 12 MI FROM TOUCHDOWN. AT THAT TIME; THE 737 WAS SLIGHTLY BEHIND AND APPROX 300 FT BELOW US. I THEN SAW HIM PROP HIS GEAR TO FURTHER DECELERATE; AND HE FELL BEHIND US 1/2 TO 3/4 MI. TWR CLRED US TO LAND ON RWY 28L AND WE SLOWED TO APCH SPD PASSING 6 MI. OUR 'WINGMAN' MAINTAINED HIS STAGGERED POS. AT 700 FT AND 3 MI; TWR SAID; 'MAKE AN S- TURN TO THE L.' WHEN I QUESTIONED THIS HE REPLIED; 'FOR NBOUND DEPS.' I TOLD MY FO; WHO WAS FLYING TO DO SO BECAUSE MY INITIAL IMPRESSION WAS THAT WE HAD A SMALL; UNSEEN ACFT OR HELI IN THE VICINITY. I THEN CAUTIONED THE FO TO WATCH OUT FOR THE 737 WHEN HE TURNED BACK. HE MADE A STABILIZED; UNEVENTFUL LNDG WITH THE OTHER 737 ABOUT 1/4 MI OR LESS IN TRAIL. I CALLED THE TWR SUPVR AND WAS TOLD THAT THE 'S' TURN WAS TO ALLOW THEM TO LAUNCH TFC FROM THE N RWYS (RWYS 1R AND 1L). THIS IS UNSAFE AND POSSIBLY AN ILLEGAL PROC/CLRNC. NOT ONLY DOES THIS DE- STABILIZE THE APCH; BUT IT CREATES A MID-AIR POTENTIAL WHEN TURNING BACK TO INTERCEPT RWY CTRLINE. SFO IS DANGEROUS ENOUGH WITHOUT COMPOUNDING THE PROB BY INNOVATIVE AND IMPROPER CLRNCS DURING CRITICAL PHASES OF FLT FOR THE PURPOSE OF CAPACITY ENHANCEMENT.