I have come across a figure that is widely regarded as the official percentage that acid bolts lower defense. If the added effet of this dagger procs and it is the same as an acid bolt procing then the defense is effectively lowered by 12.5%.
The general consensus seems to be that the acid daggers do not lower defense all that much... another option that does work however is acid bolts. As a 27-32 rng/nin levelling on IT monsters in the kazham area my acid bolts were lowering defense on nearly every hit. By the end of the fights my bolt shots would sometimes go from doing 40-50 per shot to 60-70 per shot. One time I switched from crossbow to bow after throwing crossbow TP attack and all the lower defense from the acid bolts made my bow hit about 20 points harder so I was hitting the monster for about 100 per shot. I wonder if bolts are just way better at making additional effects happen...
The defense down thing happens to me alot on things that are T or lower, and never has happen on a Hornfly or Helm bettle, a few times on soilder crawlers, i've had it where it defense down 3 times in a row but, only notice a slight damage increase afterwards
I'll add this comment. "Additional effect" weapons, in this game would be better described at "Once in a blue moon effect". You might want to take that into account when deciding
I was wondering if someone could say whether the Weakens Defense ability is a set number that will weaken defense on the first hit or if it will lower defense a bit with ever hit. I'm looking into getting daggers and dual-wielding them to see how the damage stacks up w/ en- compared to damage with swords. Thinking about using this and a better damage one. Any recomendations on daggers would be appreciated. Thanx.