If you read the article to completion you'll note that the author addresses both of your points. He only recommends it for clients who you have a long running relationship with and who thus have a pretty good idea of what your services cost. He also notes that the price generally comes out to more than what his normal rate would be, given you use the tactic in the situation he outlines. Plus he explicitly addresses clients who feel they can't judge the value properly: "just give them a quote".