Just seen some useful info about this on the Hertz UK website (obviously only applies to Hertz!)..
The Licence Summary is required for car hire in the UK, Angola, Bahrain, Balearics, Botswana, Canaries, Croatia, Namibia, Oman, Portugal, South Africa, Sweden and Tunisia.
The Licence Summary/Unique Code is NOT required for the US, Canada, Mexico, Latin America, Asia, Australia, New Zealand, rest of Africa, rest of Middle East, and rest of Europe.
I hired a car with Sixt in France on Thursday & they did not want a code. OK, I am a Sixt Express member (frequent rental scheme) so they have my details on file. Just handed over the Sixt Express card, credit card & driving licence photo card & was on my way in under 5mins.