It's really about the iteration. I would continually cooperate with the paper clip maximizer if I had good reason to believe it would not defect. For instance, if I knew that Eliezer Yudkowsky without morals and with a great urge for paperclip creation was the paperclip maximizer, I would cooperate. Assuming that you know that playing with the defect button can make you loose 1 billion paperclips from here on, and i know the same for human lives, cooperating seems right. It has the highest expected payoff, if we're using each other's known intentions and plays as evidence about our future plays.

If there is only one trial, and I can't talk to the paper clip maximizer, I will defect.