That's not true (at least not any more). You now get an account on Manning the first time you buy a book and you can download the electronic versions of those books any time from your account.
Screenshot of my account page - http://imgur.com/Hu7qT
The URL is - http://beta.manning.com
To clarify, the time limit applies only to the links that are sent via email to you. The latest version of the MEAP and the published versions are always available for download.