There's nothing "Extreme" about it. This is simply a case of solid software engineering principles. Whether you adhere to the guidelines of Extreme Programming or not, you should be taking the fact that you customer seldomly knows what he/she wants into account.
in reply to Re: They don't specify because they don't know what they want
in thread They don't specify because they don't know what they want
Update: Sorry if this seems too much like a rant, but I'm currently dealing with a situation at work where someone made an assumption and passed it along to me as fact and now I need to fix everything while we're already into the testing phase. If this would have been resolved with the customer to begin with, this wouldn't have been nearly as much work. Silly me, I should have verified.
Skepticism is the source of knowledge as much as knowledge is the source of skepticism.