Have to tell MSDeploy that it is okay for the value to be empty ...
<parameter name="SEND_TO_FRIEND_BCC" description="Email address of developer" defaultValue="" tags="">
<parameterValidation kind="AllowEmpty" />
<parameterEntry kind="XmlFile" scope"\\web.config$" match="/configuration/appsettings/add[@key='SEND_TO_FRIEND_BCC']/@value" />
</parameter>