I might have been able to sell you on it before Preditor came out. Basically a re-roll is almost as good as a extra dice. The only time an extra dice is better than a re-roll is if all the original rolls are good enough that you don't want a re-roll.
True, but a bit misleading. That is, you're right that (against PS3+) Predator and Opportunist do the same thing unless you would have maxed out your attack dice without the upgrades. But that's actually not a terribly unusual occurrence: with 3 Attack, for instance, it happens 12.5% of the time without an action to buff your attack and 42.2% of the time if you do have an action. And that means that, when compared to Predator, Opportunist is always more effective--and it's drastically more effective if you have another action (and/or token) to back it up.
Now you have to balance that against the fact that Predator is triggered with every attack, doesn't cause stress, and is a point cheaper. But to me it's not a terribly clear-cut decision.
It's useful where you're almost garuanteed full normal hits.
Remember that even in normal games, people don't necessarily have a focus token every turn, and you can pull opportunist 'on spec' without having to commit an action to it, and that you can exceed the normal damage limit instead of just getting closer to it.
Along the same lines, just seeing Opportunist on one of your ships has an effect. Your opponent will be less likely to spend actions on things other than focus/evade (like barrel roll, boost, or cloak)--Predator, and other offensive upgrades, don't have a decision-making component and accordingly can't change the way your opponent flies.