I wonder how often you use Poison/Weapon-poison as a GM in your gameplay? I dont mean for the big villains, i mean for the normal bandits on the way from one city to another. In the Geographia Aventurica are plenty poisons described, which are not forbidden and i think the healer in the party has often no use for his talent to heal poison.
Giving poisons to regular bandits often doesn't make sense ingame. Poisons are often quite difficult to produce and really expensive while bandits are most often forced to do so because they have no other way to get food or money for food (or can't do something else). Someone who can produce poisons has the talents he needs to do something legal with it, and if the bandits have the money to buy poison they more likely use this kind of money to live instead of wasting it on such stuff and risk their lives robbing people...
I agree that deadly poisons are unlikely to be found in the hands of a regular bandit.
But given a bit experience in robbery the bandit will have faced the proximity of death,
meaning he wants some "insurance",
something helping him in case things get unfavorable,
in a sense that someone scratched with his weapon (prefered) an arrow is unlikely to be able to follow him.
So what to do,? Fighting skills are not the best, equipment is bad, luck (Phex) is not always on your side.
So he might remember something like:
His parents telling about never to eat the following plant,
or just to eat some parts of a plant properly prepared,
the bandit might have also experimented with some plants effects when he was young,
or eaten somthing wrong due to hunger and survived suffering,
or be aware of some poison which is used on a regular base to poison animals,
- for example as in reality fly agaric was once used, if I remember correctly, cooked out in milk to catch and kill flies.
So the bandit may definetely be aware of poisonous plants and mushrooms and
depending on the background trying to extract or concentrate the poison by cooking might seem as a resonable idea to try.
The effectiveness of the such created poison might be checked on an animal.
Depending on the result of the check and the character of the bandit he might then poison his weapon before an ambush .
It may be far less effective than the product of an Alchemist, unlikely to kill someone
(which is usually not intended anyway, since many of them are interested in robbing not killing),
but might suffice to take the victim (at least partially) out of combat and bind in addition forces that try to help victim.
So I would conclude the regular bandit has in principle the knowledge and the means to create something poisonous,
if he considers that option or not depends on character and circumstances, if the created poison would work as intended, depends on luck and background,
so I would think of a 5 to 10% probability of poison use and going back to the original question as a GM
I let bandits at some time (randomly with low probability, trying to be fair but not predictable)
use a poison if it seems reasonable.
Poisons actually used in combat usually have to be very very potent. Even many venomous animals poisons ain't really that good at that and you got something in the range of half an hour to six to safe your life (less in Aventuria though, granted).
Less potent poisons could indeed be possible, but i highly doubt their use in combat.
They still require a (comparably) high effort to produce, given the dire circumstances bandits are in. And that for a nearly non existent benefit during combat. If it doesn't help you to outright kill your foe- why the hazzle?
Making use of the local fauna's poison however might indeed be possible. But then again- if you are capable of extracting potent spider poison- you either would not be a robber in the first place and/or would surely prefer to sell that.
However with the original intention to make a good use of those specialized healing skills- there are other ways. Not everything has to be in combat. Doesn't most RP happen out of combat anyways? It might even add way more to the "flair" of the adventurous life:
Take some of the more famous negative traits for instance.
Fringe group: Maybe the maid at the local tavern simply outright hates mages? And is not too shy to put some minor poison into his food? Nothing that kills, but maybe causes some stomach pains or even diarrhea. In the later case it would even be quite interesting (and entertaining) to figure out whether its actually poison or an illness!
Did your elf on her special diet just eat cheese? Seriously? (...)
Maybe your warriors "enemy" just managed to slip some "happy shrooms" into his dish? Or what about your groups cooking skills? Was that fish from two days back really still good? -> Food poisoning!
And of course there is still the failed attempt at gathering herbs- or the random encounter with giant spiders and venomous snakes. There ain't only bandits out there
The poisons you make I would mostly leave to the players and special NPCs to use.
If the healer needs something to cure Id put some animals or plants in to poison the players.