Benefits have costs...
At TechCentralStation, Sandy Szwarc has an article on methylmercury in fish. Methylmercury is very toxic- but as with any toxin, the dose determines the effect. Szwarc points out that proposed emission standards might reduce methylmercury levels in seafood by as little as one-tenth of a part per billion.
There is some reason to question whether current levels of methylmercury in fish are dangerous to people or children in utero (for examples, follow the links in Szwarc's article). I'm willing to grant that if my wife were pregnant, I would suggest she eat fish that has the lowest levels of methylmercury. In the presence of perfectly good alternatives, chosing lower methylmercury fish is pretty much cost free.
However, if it is true that enacting new expensive mercury emission standards will reduce methylmercury in fish (and elsewhere in the environment) by so little, then the expected benefits don't justify the costs. Emotionally it can seem that any reduction is a good thing. But paying for something one place means that there isn't money for something else, so we should be circumspect about paying too much for too little benefit.