While slaying demons isn't permanent, it can be a good thing. After all, the demon that the Matron Mothers summon may be one of the few of that caliber, and slaying it may rob them of one of their greatest weapons for a hundred years. After all, some Not a long time for a Drow, but a lot of time for we shorter-lived humans.
Slaying demons can also be a bad thing. Killing an eternally imprisoned demon, for instance, would allow it to return to this plane after a while and wreak all sorts of havoc here. ( = not good.) Not sure where the 100-year rule comes from, but it's buried in D&D texts all over the place, even in Salvatore's novels.
That's where I heard the 100 year rule, as well. Salvatore also mentioned (through a cleric of Denier named Cadderly) that the one who banished him was the only one who could summon him back prematurely. They recalled Ertu early to find Drizzt's father.
