ACR ISSUED LOW ALT ALERT BY TWR.

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AFTER 12 HR PACIFIC XING; WE WERE VECTORED FOR AN ILS RWY 28R APCH AT SFO. NORMAL VECTORS WERE GIVEN FOR INTERCEPT AT 2000 FT (GS1A 1800 FT). WX ON ATIS RPTED AS 800 FT AND 4 NM. AFTER LOC CAPTURE AND NORMAL TRACKING; I OBSERVED GS CAPTURE; ANNOUNCED IT; AND SET MAP ALT IN MODE CTL PANEL. THEN GIVEN FREQ CHANGE TO TWR. WHILE CHANGING FREQ; AURAL ALERT SOUNDS. GS UNUSABLE BY AUTOPLT. ACFT CONTINUES DSCNT AT STABILIZED ATTITUDE; APPROX 800 FPM DSCNT. I CALLED FOR PF TO DISCONNECT AUTOPLT; I CYCLED FLT DIRECTORS OFF THEN ON AS REQUIRED AND REARMED APCH MODE. ACFT DSNDED TO 1200 FT MSL (550 FT LOW). PRIOR TO CLB TO GLIDE PATH; WERE ALERTED BY APCH CTL OF LOW ALT ALERT. ON BREAKING OUT WITH RWY APCH LIGHT IN SIGHT AT 600 FT MSL; I OBSERVED ANOTHER B747-400 HOLDING BTWN RWYS 28L AND 28R. OBVIOUSLY; THE ILS WAS NOT PROTECTED (NOT REQUIRED 800 FT/4 NM OR BETTER) BUT EVERY ACFT LNDG SFO WITH A 1200 FT THICK OVCST WAS RELYING ON THE ILS COMPLETELY. I BELIEVE THE 800 FT/4 NM RULE IS NOT VALID. IF AN ACFT MUST PENETRATE AN OVCST LAYER ON AN APCH; THE PRIMARY APCH FACILITY SHOULD BE RELIABLE; IE; PROTECTED.