A lot of Notorious Monsters, possibly all Notorious Monsters added from Abyssea onward, have a fair amount of Breath Damage Taken -%. I have no idea why, but this sucks for Blue Mage and it sucks even more for Dragoon.
If nothing else, Wyvern breaths should be separated from the pool of stuff affected by Breath Damage Taken -% because using a breath is a Wyvern's only option. Wyverns are just too T-Rex-like to hold a Chatoyant Staff in those little claws, and even if they could, I've never been able to teach any of my bros' Wyvern-bros Charged Whisker no matter how many times I use it.
Normally, Dragoon only loses out on a little added damage because of this. During the new two-hour ability or a highly questionable attempt to use Deep Breathing offensively, the loss is more pronounced. Still, this is the only offensive action Wyverns undertake other than auto-attacking, so every little bit matters.
Blue Mage...doesn't lose out on much because other available options are already better options. This is part of what makes the Breath Damage Taken -% of many Notorious Monsters confusing to me.