"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".
Exactly...
Who reads email headers? Maybe some server admin who's going through some backups of emails looking for something. He might even appreciate having a date format that's lexicographically ordered for his searching purposes.
Having a human readable date is like changing HTTP Content-Length to say "one million four hundred and fifty thousand and sixty four" so that when I'm reading through my raw server logs I can easily see the magnitude of the length of the responses.