[kuchel]

I generally avoid investing in poison damage in general.
The only thing I think is worth it (and absolutely essential) is Mandrake and Nightshade.

Volley should poison all the targets because each shot is still technically one of your attacks, with damage based on your character/items/skills.
The skill you mentionned where the arrow scatters doesn't have poison isn't one of your attacks as it isn't base on your damage, it's based on the skill's level. On top of that, it's a skill that activates after you hit a target rather than something that you shoot directly from yourself (which might explain why Throwing Knives carry poisons fine but not this skill).