Distracting and compromising the immune system of a large animal while taking advantage of the free resources in the environment would probably have a very strong evolutionary advantage.
Or it could be more like the benefit yeast gets from producing alcohol, which it resists better than most other microbes.
There are bacteria that are poisonous, and there are bacteria that aren't. Clearly, both strategies are viable. So I think any explanation has to explain not just that poison is an advantage, but explain when it is and when it isn't.
Or it could be more like the benefit yeast gets from producing alcohol, which it resists better than most other microbes.