My goal when interviewing candidates is to ellicite this response at least once. I want someone who is honest that they don't know the answer and won't bullshit me about it. Also like to hear how the candidate would go about figuring it out, e.g. what resources they would use. I'm also fine with speculative answers as long as they're clearly stated as such, "I dont know, but I think it'd be something along these lines..."
Yeah, in an interview, I would certainly expand on that succinct motto; and attempt to do exactly this. But, as others have suggested, people respond too much instead of just thinking about a problem/issue/comment/etc. from all the angles (this skill was natural for me; but honed during the Ph.D. process.)
That same nugget was in The Pragmatic Programmer which I recently have been reading. In regards to estimates or questions about things you don't know, if you can't give a good answer on the spot the second best answer is "Let me get back to you.". The bad answer or estimate will just cause pain for your team/org.
