We're using one in UK, even though it is not compulsary, because it is so easy tfor someone to walk into ( or drive into) the overhanging bit of our 4 bike Fiamma rack. Yes, I know people should be careful and not go so close but they do - in supermarkets, service stations,and on campsites - and I don't want to be responsible for some child - or even adult- banging their head or putting their eye out on our bike rack.
I'm sure they are the law in Italy and will rapidly become so in other countries I suspect.
G.