I must have misunderstood 'no benefit' etc.
it depends on what 'the poisoner' wants to achieve, if its fear in his henchmen, I'd say it would probably work, wouldnt you?
alternatively, 'the poisoner' simply gets off on having the power, and the means to do this, and chooses what most would see as 'minor' irritants, to do it to....
I dont know, I didnt do it......
Presumably as a weapon it was going to be used against say a military unit, of hundreds or thousands, that fact that when used against 1 or 2 people, there is a 60-80% survival rate is down to the medics, and probably the huge resources used on these few patients. Against a thousand soldiers, you would lose a lot.