My site allows only logged members to send greetings (renamed winks). I worked on the email templates too, and the HTML notifications sent are quite nice.
Visitors even don't see the actions block on the members profile page. That can easily be done while setting correctly the actions allowed by membership.
Greetings is a greet feature ! and is a nice way to enter in contact with new friends and, by the way, increase the internal traffic of any site :-)