Do you think they picked him in hopes of winning his loyalty after the merge? I too was puzzled and this is the only explanantion I could come up with.
Unless they want him to help out in the reward challenge, he is big and strong. And I think they wanted to avoid Candace because some already suspect an alliance with her. For all the "intelligence" they gathered at tribal council, Nate willl have the opportunity to learn more, he will be with them longer, hope they are careful.