I am trying to describe the nature of pesticides. Although they are mostly harmful, we need them in order to produce the amount of food the world requires. Is there a word or phrase besides "necessary evil"?
Almost all aspects of life entail risk. We do risk to benefit analyses (albeit very poorly in most cases) and decide what is an acceptable risk, overall beneficial, an acceptable hazard, a comparative necessity, a calculated risk, a valid risk, a net positive (merit assessment), etc.
All these seem clumsy to me compared to overall positive risk-to-benefit ratio, or, colloquially, a necessary evil, or even a mixed blessing.