Following up on a post we did early in which we discussed the smart side of allowing page rank to flow in your links with do follow tags, we wanted to share a video which a Google webmaster discusses why allowing dofollow commenting can be dangerous. We share this discussion with you for this one main reason, if you do not moderate your blog comments or have some spam fighting techniques, it will not be wise to allow your links to flow page rank.
Also, when you have a comment on your blog, make sure you check the link they is being shared. If the site looks unhealthy in any way, use your discretion to disallow the comment (or simply disallow the links). Comments are important to the value blogs deliver, but it is not worth degrading the value of your website to receive a few extra comments.