This question is “old like the World”, we use to say in my Country. How can we trust of a secrecy protocol or implementation that the worldwide community can’t study, test and modify? Who may assure us that the rules of the game are clear and there’s a full respect of them?
When we are going to use a closed encryption protocol or implementation for our communications we can only trust on the service. It’s like telling a secret to our best friend: we choose someone we trust and we tell him our secret but, in fact, who knows if he will respect our privacy. Maybe our best friend may approach his cousin and say: “Hey, can you keep a secret?” and obviously the answer is “Yes!”.