I'm not sure I was clear enough, I understand having a perfectly suitable name like, lets say Edward, they can then choose to be called, Ed, Eddie, Ted, Teddy, Woody, or Edward - OK that makes sense. It's a name with options.
But for a parent to say I want to call my child Fifi, but that is a 'stupid' first name, what can we put on a birth certificate that would make Fifi an obvious nickname for them, with no intention whatsoever of ever using the name on the birth certificate? It's one thing to give your child options, but it would seem a little odd if at a certain age, they then want to known by a name never used, they might as well choose any other name under sun for what it's worth.
A number of members in my family, not my generation though, go by both their first name and middle name. My Nan for example was called by her first name by her Mum and second name by her Dad, when she got to whatever age it was, she decided she liked the second name best, but she had always been known by that in some form.