FTA: "Why not make the problems harder and let students use every possible tool or resource to solve them? Even students singularly focused on learning for its own value would get so much more out of the experience."
From my own experience my open book exams in Physics were ridiculously difficult. We would use all sorts of tricks to cram as many equations onto the double-sided piece of paper we were allotted. I feel if you agree to an open book exam the difficulty of the exam can (and should) increase dramatically. Be careful what you wish for ;-)