Succubi are tricky because - in 4e, at least - devils and hence fallen angels.In a very real sense, 4e gods are living concepts. Something about what hope IS, exists in Bahamut and in Pelor. Something about what storms are exists in Kord. Etc. When you kill Tiamat in the end of Scales of War, you aren't JUST defeating a powerful supernatural force of evil. You're literally making greed, envy, and malice less impactful in the world. That doesn't mean greed will cease to exist, but it does mean that her death should herald an age where the things she embodied are weakened:. Charity, kindness, and benevolence will flower in the wake of her destruction. These could, of course, eventually become problems in their own right (e.g. these good things being warped into extreme and oppressive things), but at least for the time being, vice will be diminished and virtue will thrive.
You wouldn't get that kind of result from killing a "mere" fiend or celestial. Kill a powerful angel and sure, the forces of good have lost a powerful member, but you haven't damaged the cause, you haven't hurt Good-ness itself. Take out a succubus, even the queen of the succubi, and you'll certainly cause a stir and probably weaken Abyssal ambitions due to the resulting infighting, but you won't make Evil-ness less prevalent. Killing a deity-level figure, on the other hand, has serious implications that go beyond the direct personal schemes and servants of the dead god.
But anyway, just as killing Tiamat should reduce the amount of greed and envy in the world, so killing the Queen of the Succubi should reduce the amount of lust and sexual wrongdoing, shouldn't it?