Originally Posted by
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Main reason to deliberately work on more *'s for an event eidolon is if you think you'd equip it as a sub on any of your teams, be it for stats, for the summon effect, or *ahem* distant future related reason.
Oh, there's also that whole trading in eidolons for a type of currency thing way off in the future, but that's not limited to SSR's. It just takes like 5 SR's to get the same trade-in value of 1 event SSR eidolon.
For Mastema specifically:
1. Her atk is low, so the main reason for her as a sub right now is if you really want that attempt to inflict dazzle ahead of 5 other summon effects.
2. (hypothetical distant future stuff; feel free to skip reading if you're not curious, as this won't be relevant for quite a while)
They gotta keep coming up with slightly different archetypes of event eidolons, right? We've had +character/+hp, +character/+ability, +character/+burst, the three dual +element, and the first of the Lilim series. Soon we'll see the start of pure single +element. And not too far away we'll see the start of +element/+resist (water is the first of this series). Then much later will be the start of +element/+double (again, water is the first of this series). Then later after that is the start of the series that is pure +element, but how much of a bonus it is varies (aaaand water is the first of this series as well).
So, +element where the magnitude of the bonus varies. It varies depending on how many eidolons of the matching element are equipped as subs, I think. Absolute maximum bonus is +50%, whereas the pure +element with no condition caps at +45%. Right now, there's only water and fire in this series, so thunder is something that's way, waaaaay off in the future.
Anyway, doing a quick count, there are 6 thunder event SSR eidolons listed on the wiki: Sandalphon, Ixion, Yggdrasil, Mastema, Meng Huo, and the respective Lilim. Meng Huo's from a collab event with Koihime Musou; whether we actually get that particular event is unknown. So the number of thunder event eidolons we'd have on our side can be either 5 or 6.
Though, it's not important to think about now. It's so far ahead in the future that you can push off working on Mastema until the reprint if a need arises.