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