I never figured out what their thought process was with this feature. They didn't offer a traditional inbox, and there was no real effort to educate users on how the "Other" inbox worked. They rolled the @facebook.com email addresses out as the default public email address, which suggested they had a plan, but they never went any further.
@facebook.com was supposed to make people feel that all their electronic communication needs can be met by facebook. I'm guessing, it turned out people didn't mind that much logging into both their gmails/hotmails and facebook.
I personally think that people did mind, but they minded the fugly messages interface even more. Every HTML email had to open in a modal pop-up because it would get messed up by Facebook's internal styling.
