Spoilers - plus help I don't understand the warriors of darkness storyline

[Cilia]

It's pretty simple, actually: a proper balance produces the greatest amount of raw aether when a Reflection is Rejoined, and the Ascians are going for an optimal amount. The Void probably could be Rejoined, but aside from potentially unleashing massive devastation on the Source the balance is not right to get a good return. It would be very low, if there was a gain at all, and the Ascians are seeking to resurrect or revive Zodiark. They'll try and get an optimal amount of aether out of every Reflection; Elidibus was not wrong when he said that the events of 3.4 can be played to the Ascians' advantage. The First Shard was originally going to be erased in a Flood of Light; now that Minfilia is there working to restore the proper balance, a Rejoining can occur that gives a better return.

If X amount of raw aether is spent trying to Rejoin a Reflection and you get X- aether in return (from, say, a Void), it isn't beneficial to the Ascians' goals. That's the basic principle behind why they want to keep Reflections from becoming Voids like the Thirteenth.

Hypothetically. Maybe theoretically.

EDIT
The essential paradox that arises from this hypothesis is "but Hydaelyn is always bleeding aether, isn't she? A Rejoining should be beneficial to her, right?" Well... yes, but it can be resolved with the information we have.

But [Zodiark] coveted power, and the balance was broken. Thus was [Hydaelyn] forced to banish Him unto the distant heavens, to forever remain apart. A moon bound.

Now... this is saying verbatim that Zodiark's corporeal body is the moon, and it's coming from the Goddess so there's very, very little room for interpretation. As Hydaelyn is at once the Goddess, the Mothercrystal, and the planet... it would stand to reason that Zodiark is at once the Dead God, the Dark Crystal seen in 2.0's ending, and the moon. Their crystalline forms only exist on in the Aetherial Sea as far as we know - representations of their physical celestial bodies. All the Ascians would need to do is direct the flow of aether from a Rejoining to Zodiark... not hard with Hydaelyn being abused as she is... and boom, Zodiark gets more powerful.

(His "dead" status also explains why the moon is a barren, lifeless rock. Hypothetically it could support life like Hydaelyn, but without aether to create any sort of atmosphere or lifestream... but I digress.)

Hypothetically. Or maybe theoretically.

Somebody stop me if I've jumped on the crazy train.