They are awkward sizes for the most part, and the cost is high for the weight of items you can squeeze in. The only exception to this is if you are shipping all the way across the country and you can manage a better deal than the regular Priority rate for that zone. But this is rare. They are counting on making a profit on people who are too lazy to do the math.
Additionally the author of the calculator last updated it in August of 2009 to align it with any Paypal changes, so it should be up to date. All times are GMT - 6 Hours