It's not only Paul McCartney who's known by their middle name, so was my step-father (Frederick JAMES) and my hairdresser, Lesley STEPHEN. Sure it must happen to other people aswell.
Loads of people including one of my housemates are known by their middle names. In her case, it's because her Mum decided she didn't like her given first name after a week of using it. Am left to wonder what George Foreman's children are known as, as he most famously called all his sons George