Profile->Preferences->"Always show my e-mail address:"
*give some anti-spam protection*
There is a second use of disable non-loged-in users to see anything, and that is brutalforce atempts on user acounts. Now, a *much* better method to protect yourself from that is strong password on things like admins and officers acounts.
hmmm, wonder how manny login atempts a login.php page for bb forums get from bots per day, and if disable memberlist change any patterns. If I get time soemday I might speend a few moments building a honepot to get some data.