Hi Taj, we have actually opened up attachment downloads to all users—even those who are logged out. Logged-out users just need to provide an email address, and the download starts. We conduct email verification later.
We believe that if a user does not recognize the usefulness of this community, they will probably not be interested in returning even if we confirm their email. Surprisingly, even without verification, people are providing their actual emails for the downloads. With this process change, we have tripled our email subscription rates—from 2K to 6K. We are also encouraging our members to +1 our pages when they download a file. Google +1 has become a significant ranking factor, and without these votes, we cannot rank well on search keywords.
Regards,
Sid