As far as I know, there is no standard format for sending calendar items over email. Such email items could be immediately put in the calendar without much effort on the user's part (at worst, clicking a "Yes" button). I could allow certain people's calendar submissions to be automatically allowed into my calendar ("Always" button). Granted, this requires a mechanisms such as PGP/GPG to establish authenticity of emails, otherwise one is vulnerable to "calendar bombs".
The same could be done for addressbook items.