They drop 50% of the time, which I wouldn't think would feel super rare, but knowing some of the luck slumps that we can all get in - it can be. I'll be reducing the number of scintilla necessary for the quest, which should help.
Oh! And just to make sure you are aware, the servitors spawn on both levels of the island, and don't just leave the lesser servitors around. Hunting the lesser ones can spawn more powerful servitors.
~ Kaitheel