The Ring of Fire islands were their base and stronghold for keeping the Door of Komalie closed, hence, the need for a defense system (the ether towers).
When you read the Leah Stone quote, she says the following: "This place is slowly draining my soul's remaining energy." She only refers to 'this place', and says nothing of Mursaat towers. As for the vizier's quote, I agree he's referring to Mursaat towers. I merely pondered how the Mursaat were able to construct energy-denying towers. Coincidently in a place near a Bloodstone. Like I've tried to do with the Shiverpeak Bloodstone, it seems the magical properties of the Bloodstone can be used in specific ways. Just as shards of the Shiverpeak Bloodstone can be used to increase the power of weapons, perhaps pieces of the RoF-Bloodstone can be used to construct energy-denying towers. I can't post a screenshot right now, but ether towers have what appear to be pieces of a Bloodstone inside of them.
There's also the hypothesis that the ether seals served a purpose other than being a defense mechanism, such as a means to absorb energy from local (organic? doesn't seem so, if they really absorb soul energy as well) organisms, energy then conveyed somewhere else. But this can be considered a forceful stomping into the realm of the endless speculations.