I did not know that. I have used autoreturn quite a bit, and validate the return to see if its an instant payment, and completed, and has a transaction ID, and it works just fine.
You are correct that IPN is definitely a better way to go, because it is not uncommon for customers to close their window before getting sent back to the site, and the DB being updated.
However, in that case, the merchant has the info in the database, with a status of pending, and if they get a matching paypal email, they can manually change the status to completed.
Not as elegant as IPN, but easier for many to set up.
Generally though, I steer clients to use a more robust payment processor like paypal pro, or authorize.net.