Considering that you are provided many sources during an open book test and you actually studied the subject, you would at least know where to find the exact information that you need when doing the test. Being that the case, having many different sources of information, as long as you know how to find them, is nothing but helpful.
If the resources are available to you then there will be a higher probability for you to perform well on a test because these sources provide you a wide array of information that might be statistically significant and related to the test you are trying to take.
The legislative branch makes laws, but the President in the executive branch can veto those laws with a Presidential Veto. The legislative branch makes laws, but the judicial branch can declare those laws unconstitutional.