[SNIP] I doubt there are many people with accounts with the same password but different email address, so the gain is minimal. [SNIP]
There are a lot of people, like myself, who have started using different e-mails for every site they sign up to. For example, wikipedia_aug07@example.com would be the e-mail address I would use if I joined WP today.
Maybe there are also a lot of people having beside different email adresses also different passwords(startings/endings) on each account...