It gives the child more of an option that way. I knew Jennifers who insisted on only being called Jen, but others who only answered to Jennifer. A girl named Elizabeth can be Elizabeth, Eliza, Elle, Ellie, Liz, Lizzie, Liza, Beth, Betty, Libby, etc, but Libby can only be Libby. It lets the child choose what they want to be known as, either the full version or a nickname. In some cases, it could even allow someone to have 2 names. For instance, a business woman named Madelyn may want to be called Madelyn at the office because it's more professional, but Maddie by her friends and family. So I think it's more beneficial for a child to be given a longer name that can be shortened. And it could also be beneficial for the parents because if the child is usually called by a nickname at home, when Missy is called Melissa, she knows she's in trouble :p. So I just prefer longer names because they leave more options.
My name's Brianne and some people call me Bri and I don't mind that, but I wouldn't want to be just Bri. I answer to either name and I like that other people have the option of which they want to call me.