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I've sat only one open-book exam in my life. It didn't go nearly as well as an equivalent closed-book exam. It may not have helped my the open-book exam that I found out I was sitting it approximately two days before it started.
Preparation beforehand is more important, still -- if you haven't done your revision before an exam you won't know where to look, or what to search for on Google, and for that matter won't be able to evaluate correctly what you find. So this wouldn't necessarily detract from the rigour of exams. You still need to know your stuff!
And, in the end, exams are very far removed from reality. I don't think I've ever had to recall a whole swathe of facts within a short space of time since my last exam. More frequently, if I am asked a question, then I can use a whole bunch of resources to help me answer it, with innate knowledge helping me to work out which resources to trust, etc.
I suppose the risk is that people lose the innate knowledge, which is still important.