There are people like me which have policy of no byte lost. You have no way to enforce on a system that user has root access deletion of anything. So if you want the mail you send to be deleted - you cannot force it.
It also adds little value to the user - if you want autobot@spam-factory.com messages to be deleted in 3 days it is the job of the client - this is a simple script with simple rules.
But lets take the "classic" example of special offers of the week ... the assumption is that after the expiry it has no value for the consumer. Wrong - the user may collect them to try to find patterns. If something is not on discount now but has been discounted 5 times in the recent year, chances are it will be soon. Think steam sales - we know that Deus Ex: Human Revolution will be 75% off, because it has been on every steam sale so far.