All this blows my mind. I've driven in all sorts of places - including most mentioned in this thread. Apart from the very first time I went to LOTFAP many years ago, when I got an IDP because 'I thought I should', I've never bothered.
I've never had any problems renting a car.
The only time I ran into a possible issue was at a mandatory-stop security checkpoint on the PanAmericana highway in far N Chile. The junior officer who looked at my documents started babbling in Espanol what was obviously an assertion that I needed an IDP. My response: a polite 'no' (no in English is no in Spanish). Into the office and he defers to his boss - all sweet; carry on senor. Essentially, they could read it.
If I were wanting rent a car where the language was not Roman-based (eg. Japan), then I would get an IDP.