IMO they have many good uses, they can be used to access your PC on the road basically anywhere arround the world! Just set the ip to connect to as a dns server that you can edit and youve basically got yourself control of your computer hundreds of miles away, like your sitting at your desk You can also transfer files from computer to computer. And other cool things.
But the bad side only happens when people get mean, and send these servers to unsuspecting people who dont want you on their comp