So every mail header in the world should represent:

  Date: Thu, 17 May 2012 08:36:04 -0700 (PDT)

as

  Date: 1337268964

because no one needs to look at mail headers and it should be the job of the user's mail reader to display it in a human readable form? I do not think it is so black and white. In the case of mail headers, the benefit of keeping the timestamps human readable outweighs the programmer cost for parsing them correctly.