Yeah, but if you are calling someone (which suggests it is probably relatively urgent anyways) and the receptionist tells you that they are going to be gone for the next week...are you actually going to leave a message?
A good receptionist will offer to take the message, but they will also offer to pass you along to someone who is in the office and can handle your request. And in the days before email, the receptionist wouldn't pass along your message until the person returned to the office unless it was important enough to call them at their hotel.
Faced with the knowledge that they won't even get my message for a week (at which point it might not be relevant anymore), I certainly won't leave one. But for email? I don't know that they are gone until after I get their autoreply...so I write up the whole email and send it. Sometimes I will then follow up with a "Sorry, didn't realize you were out--I'll follow up with XYZ and you can disregard", but that just leaves them with two emails to read...and if they read them in order, they won't see my "please disregard" until after reading the first mail.
I don't know that deleting everything is the best way to handle this, but I think it is a step in the right direction. Maybe you could have some queuing system where people can choose whether or not they want to queue the email for your return...but would anyone actually use it?.