Saturn is so far from the Sun that it receives only about one percent of the sunlight Earth gets. This means that if a spacecraft needs one kilowatt of power at Saturn, it must have an array that will produce at least 100 kilowatts at Earth. Moreover, the intense cold and the radiation belts around Saturn would seriously damage, if not totally disable, a solar array. Th Cassini RTGs have been designed to survive a wide range of postulated accidents. The goal has been to make the risk as low as practical.
Careful attention is paid to such missions so that there is no undue risk.
The rewards from missions like Cassini include giving us a better understanding of how the Solar System formed and how we came to be. How one assesses the rewards of any activity is a personal judgment. The American philosopher Morris Cohen once asked whether we would accept a magical chariot that would take us anywhere if the price would be 30,000 lives lost every year. My own assessment is that the rewards of exploring Saturn are, worth the incredibly small risk associated with Cassini. As a member of the Sierra Club and a contributor to a number of environmental and humanitarian organizations, I am more concerned about the dangers of international terrorism, ethnic cleansing, proliferation of nuclear weapons, and environmental degradation.