Certainly for hiring a car in Taiwan (which I did at Easter) the car hire company was most insistent that it would NOT hire us one without a IDP. In Australia, NZ and the UK, we've had no problems with normal driving licenses, however, with the HK one having no photo, that can require some explaining when you hand it over with your ID card, because they can't understand that it's the combination of the two that works. In that case, an IDP is just easier, imho.