I used to be more "strict" in my belief that formal names should be given and nn's used if desired. But there are some exceptions to that, I believe. Some names, that used to be seen more as nn's, are now used more often as formal names and they don't bother me BUT some others still do.
Okay:
Jack,
Lucy, and I can't think of anymore right now. :b Hopefully you get my point, lol.
Not okay:
Tommy,
Ricky,
Joey (these are actual people I know, and their "formal" names...btw,
Tommy goes by
Thomas and that's not even his real name, lol!)