For those who know more, my impression (which could be in error) is that this Captain was being trained during OE?
If so, for OE (Operating Experience), the term used for training a new pilot on an aircraft, (after simulator checkout), SFO is the perfect place to go with Instructor Pilot supervision. This is to ensure the new pilot becomes familiar with the gotchas and can get in and out of an airport without "issues". It does not matter if you've been there before in a different plane. One a new plane, it's a new checkout even to airports you been to before. If your not familiar, OE comes after the pilot has been qualified in the simulator which is considered as good as the airplane. However as an acknowledgement that the simulator is not real life, pilots new to an aircraft are accompanied by an instructor during their on-aircraft checkout (OE) to be sure they have their heads on right.
From what I've read, I'm not sure if the person in the right seat was a Captain, a First Officer, or an Instructor. Based on U.S. standards I assume he was a Captain Instructor and his sole purpose was to make sure this new Captain gained the experience of operating into and out of SFO without getting into trouble.
SFO is located in a congested area. The approaches to the West for SFO are offset ILS LDA PRM Approaches (Instrument Landing System, Localizer Directional Aid, Precision Runway Monitor) which not inherently dangerous, have some factors that can be dangerous, such as converging flight paths, and are more complicated than standard ILSs. Although I don't know if it is required, it's not unusual for pilots to receive specific training for specific airports. SFO is one of those airports. These approaches can be conducted in VFR conditions. In addition there are separate chartered visual approaches. This explains why you'd want a new Captain to be accompanied into SFO with an instructor during the OE period.
Either OE is when this would happen or depending on the airline, a Captain would not be allowed to fly into an airport (with known congestion or terrain issues) until he was checked out for that airport. This is usually done with an Instructor flying in the right seat or in the jump seat. There are several mountain stations which are considered dangerous and usually require some kind of a Captain checkout.
However, that said, based on what has been reported, (I missed the news conference), I view the accident as pilot error, possibly fatigue,
and not because SFO is a difficult airport to land at. There maybe more congestion and distraction, but the mechanics of landing the airplane are basically the same as anywhere else. It's very basic for the pilot flying to keep the airplane on speed and on a desired approach path (vertical and horizontal parameters).
The instructor pilot, the guy really responsible is more accountable than the Captain trainee for keeping the fight within parameters at all times, especially during approach. Deviations occur when something disrupts normal habit patterns, normal habit patterns are not firmly in place, aircraft malfunction, or just not paying attention, possibly due to fatigue or slow to see a deviation developing.
In the absence of mechanical malfunction, instructors are supposed to be the sharp ones who will not allow this kind of deviation to happen. Even when tired, the approach portion of the flight usually raises pilots to a higher degree of awareness due to the demands of being in a position to land and avoiding a hard landing, something pilots really hate to allow. This accident is well beyond this, it's a stall on approach followed by a crash landing. If they had been a little higher when it happened, they might have been able to do a go around or there could have been no survivors.