One thing to be aware of hen switching to "dofollow" is the increase in time you will need to spend moderating comments. You will have an increase in anything from "thin comments" (those that add little value to the conversation and are purely about creating that dofollow link), and particularly annoying spammer comments with nothing more to say than "nice post" or "thanks friend".
If you go this route, I also recommend using the Comment License plugin which adds text just under the comment form to which you can add approval criteria for the comments. By being explicit about selection criteria, you'll get a little less worthless spam.
wordpress.org/extend/plugins/comment-license/
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