When an email is composed and an address is selected from the address book, the address is in the form Nickname <name@domain.org> and is sent as such in the outgoing email. I use nicknames as shortcuts and would prefer that not the nickname but the name is used in the To, Cc and Bcc fields: Firstname Surname <name@domain.org>.
One reason is that there is only one person with a given nickname, but many with some names, so the right email address would be selected faster and with fewer errors.
Also, odd situations may happen, typically when writing to a family member and a stranger in one mail. You do not really want that person to see the nick you use.
What do you think?