This reminds me of one of my favorite threads on the old Internet Infidels web site, in their Philosophy forum. The question was, "Do dogs bark?" There was an enormous amount of discussion on it!

Does "when pigs fly" imply that some day pigs will be able to fly? No; people can understand impossibility when it is used rhetorically in every day speech.

For example I might say, "all the honest politicians are doing a great job", which conveys my actual meaning, "all politicians are dishonest".