I think the point of the flat parts is that if the LEDs get fully inserted to the epoxy case topuching the circuit board, overheating is more likely, especially during something like hand soldering, where the time of heat applied to the leads may be longer than the heat it gets during wave soldering. Also, having air space with exposed leads would probably give a lower operating temperature for the LEDs, which gives a longer lifespan since heat kills LEDs.