Just had my first problem when debugging an external email. Reading the mail via IMAP I couldn't see all headers and since I didn't know which x-headers would be interesting I couldn't send a mail to promote it. I just needed to see the original unmodified message as it was when it passed through our anti-spam filter. This will be a big problem for me in the future when debugging emails.
Another question I'm waiting to get on my table regarding this change is how we can let users moving to other providers get a full copy of their messages with all header? Before we could use different "imapcopy" tools but that will not be possible in the future. What would you recommend in that situation? pst-files is not an option.