It has been told to me that the stiffness determines the power a racquet. The stiffer the shaft the more power. If you look at it like golf club shafts you need to swing hard enough to get whip action from the shaft. Is this not true of badminton? One needs to be able to swing hard enough to get the most out of a stiff shaft. If you swing slower you would need a more flexible shaft.
Joey, read the thread "String tension again", the same theory desribed there also apply to the shaft.