Its better to think of the rattlesnakes in terms of ecology rather than the idea that they should serve some sort of human benefit oriented purpose. Their historical range has not changed very much, but it is thier preffered habitat in SE wisconsin that is changing. SE wisconsin used to be predominatly prairie and oak savanna before the settlers settled here. Both of these habitats are the preffered living space of the timber rattler as well as deciduous forests. Wisconsin is now 50% forest due to conservation efforts. In SE WI a lot of land owners and organizations are converting their land back to what it used to be historically: prairies and oak savanna.
Due to the conversion of land to that of the historical landscape, I would presume that SE WI could become part of their natural range. They are probably planted because these "wild" areas being converted are broken up by cities and suburban areas. They would probably not be able to disperse naturally on thier own due to the "islands" of habitat that exist within our cities.
As far as saftey goes, people would have to be careful and hospitals would have to start carrying anti-venom. Perhaps the DNR would have to make an announcement about snake saftey to the public.
We can't just extirpate the snakes after they have been introduced based on human saftey. With all of the land converted, certain animals would dominate. In prairies, small mammals and birds would dominate. Without any predatory control other than hawks, the ecological balance could be compromised.
I also think that WI needs more wild places and native plants and animals.
It would be a battle between human saftey and ecological integrity, but that is for the DNR biologists and the public to decide.
Just my thoughts, Fly.