Suzeman
There is a marked difference between declared capacity and achieved rates. Achieved rates were regularly well in excess of 50/hr in the late 90s with a record hour of 60 between 8 and 9am one fine September morning.
Declaring 44/hr and adding ATC flow control into the mix, often meant that departures from the 7-8 hour were pushed back into the 8-9 hour when most of the arrivals were scheduled. With just one runway, this meant lineups for departure and airborne holding. Airborne holding was more regular and prolonged back then, so there was definately a perceived benefit to having two runways even if the capacity didn't go up much.
Magplug
Just because Gatwick doesn't use two runways shouldn't mean that anywhere quieter can't build a second one. What's the magic number?
IMHO the runways shouldn't have been built with the stagger but I'm not a planner just a worker. All over the world parallel runways are built closer and not offset like this (e.g. YYZ 1000'/300m centreline to centreline vs MAN 390m). This would have made a difference in that you could always land on the outer and depart the inner. You'd also have taxiways to the end of
both runways.