Question in my head is: for an A320 with four hours of FOB, does it really take three hours to burn off enough fuel to meet the conditions of the FARs?
Thats a very interesting question. More interesting is "does it take 3 hours to get below maximum landing weight".
Gabriel has summarised it quite well.
At the end of the day, there is a weight at which you can land the aircraft, and a normal maximum landing weight. Obviously very different things.
For a start, I'd suggest there are very few aeroplanes around these days which *require* a fuel dump system. Many have them - but don't necessarily require them.
Its also a case of balancing risk. Back to what pilots are paid to do. What is the greater emergency/risk? If you are on fire, down to your last electrical generator etc, then the risk associated with not landing is greater than the structural risk to the aeroplane.
Loss of one hydraulic system? Not a reason to land overweight (unless other considerations apply). In many cases you would continue your flight. However, having probably been startled by the problem with the second hyd system in this case, the crew made the decision that while they still had two hyd systems (no reason to land immediately), it was probably a good idea to remain near an airport where they could put it on the ground immediately if they got any signs they were losing the second system again. Pretty smart really.
Again, it depends on the aeroplane. The A380 has specific provision for overweight landings in certain circumstances. Other aeroplanes really don't like it. At the end of the day, as long as it is a "normal" landing very little will usually have to be done to clear the aircraft for further flight. In some cases, nothing. Max landing weights are usually about maintenence inspection times and longevity of the parts. Case in point: you can often "buy" higher max takeoff and landing weights from manufacturers.