So URL's mentioned in comments receive credit. Looks better to search engines (more backlinks). Google, unless I'm mistaken, doesn't count 'nofollow' links as valid or legitimate backlink.
Blog owners turn off nofollow so as to encourage commenting (because it rewards the commentors). That way, it's a win-win situation because the dofollow blog gets more traffic and loyal visitors, while the visitors get backlinks. Nofollow isn't really able to reduce spam - people still spam even when most blogs have comments that are nofollow.