Today I changed the configuration of this blog to dofollow. For my point of view, the web is a space of sharing and if a visitor adds a valuable comment to something like a blog, the least that the blog owner have to give in return as a gratification is an ounce of Googlejuice.
Nofollow attribute is, in my humble opinion, the worst way to discourage spammers. Except your satisfaction that the spammer didn't get any value in his activity,there is no other advantage. It doesn't discourage spammers and doesn't stop spams in your blog.
So, I am deactivating the nofollow attribute, and will monitor manually the comments. I will be removing comments which don't bring any value, especially those with an hyperlink. I am also planning to make a sort of blogspam.net proxy, to fight spam my way. This proxy will act as an URL blacklist checker, so I will be logging any domain mentioned in a spam comment and reject any further attempt to link to this domain. If the spam passes my check, then this module will forward checking to blogspam.net, before accepting the comment. A final manual review will be performed and possible spams will be deleted again.