The voice synthesis is really nothing new, it has been part of iOS as VoiceOver (Apple's built-in screenreader, an accessibility feature) for a long time. It certainly wouldn't have any problems reading emails and it seems to me that emails tend to be less random than text messages.
Maybe they wanted to keep the interaction short to give it more flow? Siri doesn't really ever seem to read very long stuff to you.
True, I guess the modern language used in text messages could cause problems, however with the lengh of signitures and message history indentation that I get in a lot of emails, I could see them being an issue.
