Firstly, an open-book test is to let students do what they know. If something slipped their mind, they may refer. This will hence reduce the stress of students being afraid of them failing their tests, while at the same time letting them learn and know what they have already learnt.
In my opinion open-book exams are a good approach to testing. It decreases the stress incurred through testing for someones knowledge and as well provides a learning opportunities for the people taking the test. It can be a great way to expand capacity and bring in new ideas, which is a boost to creative thinking and positive development.
If the student can just look up answers in the book then there is no need for them to actually learn the infortmation, which defeats the purpose of our education system. Sure it would reduce stress but school is not meant to be easy. School is meant to challenge you so you can learn new information. When you are working a job you need to have certain things memorized, you are not going to be able to look everything up in a book.
Although open-book exams tend to yield higher scores in general, I do not feel that having the exams be open-book enables the student to learn the material properly. Good notes are a guide to memorizing the subject material well and being able to call it up in your head. Open-book exams are a bit like cheating and the content does not stick as well when it comes to having to do close-book exams.