I tend to think that acts of suicide terrorism help sponsoring groups achieve their long term objectives. The first reason would be that the suicide terrorism directly validates the insurgent group. The sponsoring group is able to point to the suicide terrorist as representative of how the group is committed to their particular message. At the same time, the person willing to act enhances the group through their action. The group does not have to worry about continually seeking to appease the individual or does not have to worry that the individual will betray the organization. Once they die, another is sought to take their place. This helps the organization continue and allows the message to resonate.
At the same time, I think that acts of suicide terrorism helps to show how committed the sponsoring organization is to change. It helps to achieve the group's objective because it shows gravity of seriousness and commitment in overall end. Even if governments take the position of refusing to negotiate with terrorist organizations, the forceful nature of suicide attacks compels a government to have to deal with the organization because of death and destruction. This is where the sponsoring organization has "won," to a great extent. In this, their objectives have been met. The long term objectives of the organization will never be authenticated by a ruling government. Therefore, to have to be reckoned in the form of suicide terrorism is one way to have to force the issue on the ruling government or social order.