Air carrier landing at SFO voiced concern regarding NORCAL's handling of the side by operations that frequently occur; noting requests by ATC to positively identify parallel traffic at night is especially difficult.

Narrative

We were on vectors to 28L. After indicating the airport in site; NORCAL pointed out traffic on 28R. We indicated the traffic was in sight but that we would not be able to maintain visual contact. This was due to the fact that we could not assure the ability to remain behind the traffic based on speed and closure. NORCAL indicated 'that's OK; he has you in sight and will maintain the visual'. We were assigned an initial speed of 180; then 160 KTS. We were handed off to SFO Tower approximately over the bridge. We were cleared to land 28L. Inside the bridge on short final the aircraft on 28R passed us. While we did have the traffic in site as he passed and were aware of his location and runway assignment; we had not accepted responsibility for traffic separation. As this happened on short final; and the Tower frequency was busy; I am not sure the other traffic could have informed Tower of the passing at the moment it occurred. I did not feel safety was in question; and had we gone around it would have likely caused us to pass the other aircraft; as well as place our wake above him. I am not clear if the Tower accepted responsibility for separation once both aircraft cleared for landing. I feel very strongly however that this continues to be a significant problem into SFO. Clearances are issued constantly that cannot be complied with; 'maintain visual; he has you and will maintain visual'; two aircraft maintaining visual on each other. Or; 'cleared for the visual; maintain visual; maintain xxx knots'; wherein the pilot is forced to either maintain visual and not pass; while altering airspeed in possible non-compliance with instructions; or if the pilot maintains the airspeed he/she is put in a position of passing the aircraft when maintaining the visual. NORCAL also often asks to identify aircraft so far away that the pilot cannot positively identify the company and type; yet is pressured over several calls to maintain visual. This is even worse at night when it is not possible to identify the specific company and aircraft in order to ensure maintenance of visual contact with the correct aircraft. This; coupled with frequency congestion places the pilot in a very difficult situation attempting to comply with ATC instructions or separation standards.