I'm fairly strict with what I allow as comments on this blog.

But if someone has gone to the effort to post a well-thought-out comment I don't mind if they put a URL in the comment or the associated website field and therefore gain some SEO benefit from it.

I have recently discovered that WordPress by default puts a rel="nofollow" attribute into all comment URL's and that Google honors the nofollow directive thereby negating the possible SEO benefits of commenting.

By default the nofollow directive is a good thing, it devalues the link because it doesn't give the linked website a lift in pagerank with Google and therefore is only useful to a real human who has to click on it to discover it's content.

However if you only allow comments that add value to your blog, then the use of nofollow becomes a negative feature. I don't know about y'all but I like to reward good comments. To that end I have installed the DoFollow plugin which strips out the nofollow attribute from the comment links so now the GoogleBot will follow the link and discover the content giving them a SEO boost (hopefully).

In conclusion: If you are vigilant with moderating your comments, and use plugins like Akismet and Hashcash to weed out automated drivel then I don't think there is too much harm in using DoFollow.