Microtransactions are here to stay. And they are a good thing.

[Rufalus]

This kind of feature requires quite a lot of resources to be implemented, and it has quite limited influence in the success of the game and in the revenue it'll bring to Square Enix in itself. The ability to attach some absolutely optional microtransactions to it is probably what got such a work/resources intensive feature greenlighted to begin with.

This is the best argument I've seen for this feature. Without the optional premium tier eternal bond, it's possible they would not have been able to justify putting any resources/time into it, and thus the feature would not exist at all, even the free entry tier.