First, the recipients will not see an email address in the To field of their email. Also, you can't personalize email when you send the same message to everyone. These are minor setbacks that are usually acceptable. Keep in mind that we would probably not want to set the To property to specify all of the recipients. If we did, all of the recipients would see the other recipients' email addresses.
Most people consider that an invasion of privacy.
Most people really don't appreciate getting 500KB mail with only a few lines of content. You can even attach files to messages using the AttachFile method. This is usually an OK, but annoying situation. Another issue that we have found is that the From property must be specified. The content of the From property doesn't seem to matter, as long as some sender is associated with the email.