In general I think it's silly. I don't approve of grown men going by
Billy or
Bobby or
Jimmy (
Carter notwithstanding).
In my family, my brother was called
Johnny as a child, but once an adult, he wanted to be called
John. However, my sisters and I still call him
Johnny because we can't think of him any other way. But at least everyone else calls him
John.
OTOH, the name didn't seem an impediment to
Johnny Carson and
Johnny Cash.
And then sometimes leaving behind a childish nickname happens naturally. That's what happened with my sister
Pamela. As a child, she was called Pammy. It just happened naturally that as she entered her teen years, we all started calling her
Pam. I don't know why that didn't happen with
Johnny also!
My sister
Patricia was
Patty as a child and has stayed
Patty as an adult---never shortened to
Pat. But for some reason,
Patty doesn't seem childish the way Pammy does.
Wow this subject was more complicated than I thought it would be.