I'm assuming we're talking about damage when actively tanking since having Defiance up while not getting hit is silly. Crit from wrath stacks is pretty negligible because holding onto 5 stacks is going to be a rare occurrence.
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He was talking about crit being valuable because wrath already gives us 0-10% crit. I was pointing out that the wrath stack crits do not make wearing CRIT gear any more or less valuable. And since the context of the discussion was CRIT vs. DET, I stated the effect it has on DET, regardless of how minor it may be.
But we're also continually building wrath, so wrath is probably adding 4-5% crit on average, depending on how long you hold your Wrath V stacks (when saving it for a specific move).
To expand a bit on what Giantbane is saying, because of how powerful a secondary stat Det is SE sets Det's cap to ~66% of the other Secondaries and Savage Might materia add fewer points of Det compared to other materia of their Tier. At i110 this is 31 to the "normal" Secondary cap of 44. Novus weapons can have up to 75 secondary stat points infused into them, 1 point per materia infused no matter the materia type being infused. This makes it possible to cap 2 different secondary stats on a Novus weapon as long as one of the stats is Determination. So a 44 Parry/31 Det Bravura Novus would be stronger than a 44 Parry/31 Crit Bravura Novus.