no i am putting my foot down on this
Sure, in-character Dagger wouldn't have brought it up to Zidane unprompted. But that's why writers create scenarios when something needs to be explained?
No. If your character 1) has a unique magic power that is rare and special in the setting, 2) knows she has this power, 3) is afraid of using it, then your two possibilities are "this is treated later as a shocking reveal when she tells us she has this power," or the reader needs to know this information.
FF9's problem is that it makes the reader aware of 1, but not of 2 or 3. We know Dagger is a potential summoner, but nobody brings it up until after the eidolons have been drawn, so all we know is 2 and 3. We're left to infer an answer as to why she isn't summoning and why no one is talking about it.
If this is a dilemma Dagger is facing, we need to know before she resolves it. We can't learn that she knew about the summons but was afraid of using them in the same breath as her getting over it. That's not how writing works!! If there is no in-character reason for Dagger to disclose this information, then you have a hundred solutions to this issue, ranging from "flashbacks" to "inner monologue" to "contrive a scenario in which she brings it up" to "one of the antagonists gloats about it to Zidane."
I don't care if there was never a situation in which it would have come up until it was too late because the result is bad plotting. It was the writer's job to ensure a reason would present itself so we could have the information that's relevant to us, or, failing that, actually make it a twist. Like, you could have just have Dagger say "I... knew all along I had that power inside me... Even though I never talked about it to anyone..." and then everyone acts shocked, least of which because they went through multiple boss fights where that might be relevant and because it suddenly explains why all these Black Waltzes were so gung-ho on getting her back.
But no! This is not treated as a surprise to anyone. Dagger talks about the summon power as if the characters knew this entire time she had it and no one is bothered in the least.
It's poor craftsmanship.