The assumption that a stimulatory effect is always beneficial to an organism is perhaps what troubles me most. The investigations of Dr. Holland's groups revealed stimulatory effects on plant growth in studies dealing with some of our more notorious pollutants including mercury, sulfur dioxide, ozone, disodium arsenate and xylene. They also found other studies which exhibited stimulatory effects of penicillin on staphylococcus growth as a measure by turbidity and on a wide variety of other species and chemicals. The problem here is that many of the pesticides kill by overstimulating some aspect of an organism's functions. Some of the pyrethroid insecticides kill, in effect, by literally stimulating an insect to death by inducing repeated, uncontrollable firing of its neurons.
Some herbicides literally burn up their target plants by over accelerating growth or metabolism.
Considering this, it comes as no surprise that low levels of either of these would be found to have stimulatory effects on target organisms but it does not mean that these exposures would be beneficial. There are many other examples where an agent's lethality is due to overstimulation of the target organism but in the interest of brevity, I will use one final gross example. Cocaine is an excellent stimulant but few people would argue that its use should be legalized or that we all should take repeated low doses of it.