I find "why would you want to know that?" terribly annoying, especially when it's in response to a question whose intent is reasonable and well expressed and deserves better than that kind of deflection. It seems to happen in an extremely kneejerk fashion though. Deflect, deflect, deflect! Before consideration is even given. I'd much prefer to hear something more honest like "let me think about how best to answer that" when that's what he actually needs to do.