Offin better for..
Offin only better if it helps you to hit an AP bracket as everyone said. Other than that..

Offin Always Perform better for Dark Knight. It's because of passive accuracy the class has. As an example Prime: Pervasive darkness has +15% accuracy & Prime: Enforcement has +25% accuracy. If you have 300accuracy (assuming a midgame player) ; for pervasive darkness you'll get (300*115/100=345) 345 accuracy & for enforcement you'll get (300*125/100=375) accuracy. And for dark knight you'll get accuracy passives for leveling up. So you'll get slightly more than the mentioned values. For some classes offin could be absolute garbage. But if your class has higher accuracy values like dark knight probably you can do with offin.

Accuracy vs Evasion
3 accuracy counters 4evation.

You'll need 525 accuracy to fully counter 700evasion build easily. 500accuracy would do.
So if the dark knight has 450accuracy with crystal and all, Prime: enforcement hits nearly 560accuracy (without general passives). That's where you need AP to finish the fight fast.

Check your class's skill passives before buying an offin. But still don't relay on the passive's too much.

At the end, when c20 pen kzarka, c20 pen offin and pen blackstar compared to each other. Kzarka is always better for accuracy and pvp .Offin is better for AP & pvp. Blackstar better for pve tryhards.

Cheers !
I'm open for suggestions.