People use the same user/pass combo for every site they visit, except when one of them forces them to use a complicated password that they can't remember. So they send themselves an email with the site name, username and password so that they can find it next time they need to log into their bank.
So once your registry cleaning website has their email password, you also have a nice list of all their strong passwords too.
Adding to the irony, most people know that they need a different password for their bank, so if you just let them pick one without forcing complexity, they'll choose something they can remember, and their bank account will be safe.