Well, I prefer formal names to nicknames. I generally don't like nicknames used as full names, especially the really childish ones like
Maddie,
Timmy, etc. I find they might look awkward on adults, especially in professional situations, and it's better to have a formal name "on the dotted line." Parents can still use whatever nickname they want, while leaving the child the choice to use a different nn or use the full name if they want once they're old enough to decide.
I can tollerate nns that are ok on adults such as
Tom or
Alex, but I still think it's better to have a formal name personally. And I'm ok with nicknames that properly evolved in full names in their own right, like
Nora,
Megan,
Lisa.