I think the importance of the leadership depends, to some degree, on how much emphasis the group in question places on that position. Take ISIS, for example. They claim to fulfill some ancient prophecy (to sum it up very loosly). The way they tell it they are not just a caliphate, they are the caliphate of ancient prophecy. But a caliphate needs a caliph. Taking out their leader might actually convince some of their followers that they are not what they claim to be. With Al Quaeda it might not matter all that much, but if we were talking about ISIS it might put a serious dent into their propaganda.