I have no doubt that special attacks will poison your foe if you have poisoned weapons on. But the upgrade for more damage (+25% damage) only applies to normal attacks. So for example, thousand cuts with poisoned weapons will do the normal amount of damage, but will also poison your enemies. But the little upgrade does nothing for 1k cuts, twin fangs, shadowstrike, flank attack, throwing knives, etc.
Really I only take poisoned weapons for 2 reasons...and even then it is only on AI-controlled rogues.
- Early game means to help ai-ranged rogues to self-heal and not drink so many damn potions
- A quick way to get to toxic cloud and it's upgrade for some party damage avoidance
I can see it's value for solo'ing. It's the rogue version of guard. But for groups and general play, I would never get it for my own character. I'd much rather have twin fangs and death blow. Or Twin Fangs and Shadow Strike. Or hell I'd even take spinning blades over it. Just saying there are other places to put skills that are more fun.