Other recent research suggests that decapitation can lead to more, and less selective, violence on the part of terrorist groups. Moreover, while air power is great at stopping advances, and also quite effective in killing selected leaders, it has a poor record in dislodging terrorist groups from territory they've seized.
Despite these cautions, the recent wave of leadership deaths in Al Mukalla suggests there are real constraints on a group's ability to take advantage of instability and seize territory, even in a situation as chaotic as the war raging in Yemen.