Longitudinal layout is so weight is closer to the middle of the car. Being mid-engined, it is of benefit to the handling if a big engine isn't throwing its weight from side to side as you corner. It is however very difficult to package that kind of layout, which is why it tends to be restricted to Ferraris and the like. Also, something like a V12 Lambo or V12 Aston would have to be longitudinal as it wouldn't fit in sideways!
The NSX has a dry sump which means the engine can be lowered, giving a better CoG.