An exam is a totally artificial scenario. In real life you will never be in a scenario with all these fact at same time: a) you can not consult external resources or colleagues; b) you have limited time; c) you do not known the activity before to start it. As conclusion, exams should be deprecated in all forms, with or without consultation of external resources. They exists only because they are a fast and practical way to obtain a numerical evaluation for a big (massive) group of students.
Starting from this point and returning to the question about usage of external sources of information, we can analyze some different scenarios:
a) if a concept is basic, could be it is already acquired before the exam (not the same than memorized) or student will fail the test, because he/she has no time to acquire the concept and use it only during the time allowed for the exam.
b) thus, to have or not to have access to external sources of information is only a difference in the case of specific items of data information, like some particular formulas, taxonomies or algorithms. In these cases, allow notes or books has only advantages. In fact, if something can be successfully retrieved during the exam it means it is not knowledge but data.
c) allow notes minimizes some of the usual drawbacks of the (artificial method) exams: when anxiety causes a fail in the memorization of a data term; a question that fails into a paragraph that has been ignored as unimportant; it made useless hidden notes that benefits only cheaters; ... .
d) if the student has not acquired confidence in its book, course notes and/or write own abstracts, the presence of them during the exams will not help. Thus, allow these resources encourages the necessity of write abstracts, organize sources, ...
One classical example is the periodic table in chemicals. It is the perfect example of a resource to allow, not to memorize, and instead use this time to most conceptual activities (in my country, students loss months memorizing it).
In conclusion: skip usage of exams; if exams are a must, allow books, notes, even intranet.