I like to leave comments on, can't say if its better for SEO but I'm thinking it could be if the comments are relevant.
However, when using WordPress on general porn blogs, I setup filters to auto-deny anything with URL's in it, and also remove the URL field from the comment form. If the form is submitted with the URL field filled out, I know its a bot, and if there is a URL in the comment text well, fuck it probably a bot as well.
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