If we enable sharing calendars (set the Calendar permissions to have other people as Reviewers) we then see each of those people now having the ability to send email as the sharer (eg. if I share my Calendar with Joe, Joe can now send email as me). Looking in the Delegates permissions I see no option to disable that, and they have no access to the inbox. This seems like a major issue, as simply allowing someone to see my calendar seems to allow them to impersonate me.