International Driving Permit

It is unclear to me how vital an International Driving Permit (IDP) is for Namibia. The RAC site, http://www.rac.co.uk/web/know-how/going-on-a-journey/driving-abroad/driving-abroad-how.htm, indicates that it is not essential. But then when you search for Namibia says that if you hold a green driving license then it is required. It currently costs £5.50 so we'll play on the safe side!