I tend to not want to waste zetas on a character if I'm not going to use them. If the zeta will help, then sure.
Keep in mind that if your mods are good enough, you need to only get them to g12. 6-dot mods also add more power than 5-dot, so if someone still isn't meeting the threshold at g12, you can swap mods to them just for the event and you won't need a zeta.
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Basically what I’m asking is, what’s less important, gear or zetas? As I don’t plan on using the characters after unlocked
Keep in mind that if your mods are good enough, you need to only get them to g12. 6-dot mods also add more power than 5-dot, so if someone still isn't meeting the threshold at g12, you can swap mods to them just for the event and you won't need a zeta.