>>8
E-mail field keeps out good posters. Imagine someone with an involving job related to your forum comes across it. This person is an expert in her field, and therefore would be a great source of knowledge for your forum; but if there are e-mail links, which are filled with mailto:noko, she might just give up on posting and do something more important. People with lives will tend to ignore forums with an e-mail field.
Registration lets in bad posters. On the other hand, people with no lives will thrive on your forum. Children and Internet addicts tend to have free time to fill the e-mail field with noko. They will generally make your forum a waste of bandwidth.
Registration attracts trolls. If someone is interested in destroying a forum, an e-mail field only adds to the excitement of a challenge. One might argue that a lack of an e-mail field will not let "anyone" post their e-mail, but in reality anyone can post their e-mail on the comment body itself when necessary; e-mail field is merely a useless hassle. Quoting a world4channeler:
Trolls are not out to protect their own reputation. They seek to destroy other peoples' "reputation" ... Fora with e-mail fields are like a garden full of flowers of vanity a troll would just love to noko.