A Bcc header *can* be included in the version of the message going to
the bcc'd user. *Some* MUAs split the message into multiple versions:
the ones going to the To's/Cc's do not have the Bcc: header, and the
ones going to each of the Bcc's have just that user listed. Other MUAs
choose a simpler method of just stripping Bcc: entirely and sending the
same version to all users.
And one of the wins of the bcc header appraoch is that it works with vacation
list checks. The one-copy-for-all approach does not.