Way back in the 90's, my sales manager told me the truth about references and case studies. They're useless to real salespeople. Don't get mad when I say that, I'm just repeating what I've been told, but he had a great point.
Case Studies are marketing documents. They aren't for salespeople.
When a client asks you for references or case studies, they're not asking to check on our work. They're telling you they don't believe you, and they want some measure of proof that someone else was willing to vouch for you.
Do you know why clients ask you that? It's because you're a bad salesperson.
Now, that doesn't mean that there is never a time or place for case studies, or references, or tests. It just means that if a client is asking for those things, it's because they don't trust you. And if you want to gain their trust, you don't get it by letting them look at a case study.
You have to ask them what you missed. Because this I can guarantee you - they don't ask everyone for case studies.