The key point is now. If it's a fire event on the other hand, Cybele likely won't be an option, and Belph isn't an option for everyone since not everyone has pulled her. Also to rely on Cybele, that would mean you'd have to heavily invest in wind if you want her to be effective in non thunder events, otherwise you're losing a LOT of damage by having a single unit just for a debuff.
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I understand that's why i said it is case by case. Assuming some kamihime overwrite the effect and not stacking so the effect won't be crucial either since some kamihimes already have its own debuff, Like nergal, baphemot etc. alternate option like healing, regen etc can be a good option and in some case can shine more than debuff.