It really annoys ladysolly when I say this. And I've noticed that it's not an expression that's widely used.
But the thing is, often when I'm asked a question, I don't know the answer, so I can't see what else I could say.
I think a lot of people don't take this view. When asked a question, they'll give a guess to the answer, rather than admit ignorance.
Looking at the Church of England, I feel that this is a stance they ought to take on their position towards homosexuality. Clearly, a whole bunch of them think that gay marriage is wrong (and, I'd guess, gay bishops). And clearly a whole bunch of them, roughtly to the same number, think the opposite. All of them are trying to discern the will of their god, and it's clear to me that they've been unable to achieve clarity on this.
So why don't they just say "We don't know"?