Kiwi,
Just tried to post a long reply only to find that the system dumped it!
Here goes in quick time.
Because the -500 has a shortened rear fuselage the rudder is less effective. On a short runway we have to accelerate the a/c to V1 before we get it airborne. However because of high Vmcg, V1 also has to be high and therefore we have to severely limit the weight to enable a very quick acceleration to V1/Vmcg.
By reducing the thrust we dramatically reduce the Vmcg and thus lower V1 as well.
Lower V1 means an increased ability to stop from higher weights and so with less thrust we can lift higher weight off a short runway.
Easy huh?
Lockheed themselves realised the problem and developed a fix for airlines who regularly operated out of short strips. BWIA used this mod in the Carribbean which was a fibreglass rudder extension that restored the effectiveness of the rudder.
The short rear fuselage brought with it many other problems that Lockheed had to address. Some were dynamic high speed gust alleviation (the first of it's kind I believe), over pitching with go-around thrust application and others.
Great machine though!