Honestly, any tests I took that were open book were long enough that if you were constantly looking things up you'd never finish. Same would occur if you were talking to someone and they were having to relay the info.

Part of the benefit of open book tests is that it brings up the "floor" of resource access, limiting the potential differential between honest and cheating students. Obviously, this could be negatively compensated for with other factors, like having more questions or stingy grading.