Why would or should they do it? Sure, this is kind of age question and matter of genre, but you must answer two questions (to you, not for me because I’m not interested ):
why would someone buy something without seeing what they are buying (aka. Why would an user give email address to you; they don’t know you)
why are lurkers so big issue in the situation where about 2 % of users create 90 % of content (aka. What is the real reason why you want to get registrations/emails)
And the last one: is content really so exclusive you want ban search engines?