The only tests I ever had that were either open book or open notes always had more emphasis on whether you could answer the questions at length rather then whether you could spout back the formulas or facts.
For instance you might have an open book exam in history if the questions were going to require neatly argued positions (supported by what you looked up) rather than just recall. And in calculus I had one professor allow a sheet of formulas, but you still had to work the lengthy problems that required those formulas. There wasn't going to be a question like "what's the formula for ___" where you could copy right out of your notes.
I think that would make the difference for me... If they're given questions where they can look up and then copy out the answer verbatim, that doesn't seem like a useful test of anything but their speed. But if they're allowed to reference the text or notes for supporting points that let them then work out something more complicated, that might be fine.
