As highlighted in the book in terms of the powers of committees, the issue is always whether you have what you need to fulfill your fundamental mission as the grand inquest of the nation, and whether you are doing that, exercising those powers, in a way that is not harming other public interests, public policy issues or individuals.
It's really a case-by-case situation as to what you need, as a committee, to do your work. What information do you need to be able to fulfill those functions of legislating, deliberating and holding the government to account?