I would say it is because they only have one type of damage for poison, and like bleeding, it stacks on itself in ways, and these creatures, low level or not, need a way to make them annoying or dangerous, even if only slightly.
I was concerned when I blew up a barrel in a search and destroy mission and it poisoned me for over 10 minutes, now that was cool and unexpected (someone made the joke that I blew up their toilet and was standing too close). But why poison? Why not radiation in that instance?