This site spells it out: http://www.snopes.com/inboxer/scams/license.htm

We were stopped in Cogne, Valle d'Aosta, Italy, where almost no English is spoken, by a couple of bored-looking cops. We were glad to have the IDP so they couldn't complain about the fact that we didn't.

The rental car companies aren't at all interested in whether you have an IDP or not because an IDP is not a license to drive a motor vehicle.

Law enforcement in some countries (or in areas less touristed) may use the fact that you don't have an IDP as an opportunity to give you grief.

For the few bucks it costs, we do get one when we are driving but Tom's idea of changing the date on an old one is a great tip!

Cheers! Don and Linda Toronto