We know for a fact that DDT has a serious impact on wildlife. Humans aren't special. If something is causing tumors in wildlife as DDT did then it will cause increased cancer risk in humans as well. And studies have shown that DDT travels up the food chain to higher animals as they eat lower animals, gaining more and more concentration each time.
Since humans are the ultimate apex predator that eats other stuff on this planet we need to be really careful about the chemicals we release into the environment, unless we want to end up poisoning ourselves. It's not complicated: creating and dumping large amounts of a long lived chemical that does not break down easily, and which causes increased cancer risk is a really bad idea.
