Many bloggers accept all comments on their blogs without knowing that search engines penalize you for posting spammy comments or irrevelant comments from spammy sites. They don't want the users they send over to your site to be dissatisfied with non relevant stuff on your blog.
Use comment spam services like Askimet. Otherwise the best thing to do:
1. Moderate all comments to your blog
2. Read through the messages and post only those that seem genuine
3. It’s preferred to have comments only from those who sign in
4. Allow those who don’t want to sign in to post comments, but choose only those that add value to your blog or provide readers with more information
5. It’s preferred not to accept those who post nothing useful but leave their signatures
6. It’s best to check the site of any spammy looking comment and find out what the site is offering before accepting the coment.
What’s good about comments on your blog?
1. They provide user generated content
2. They tell you how useful your blog content is
3. They also let you connect to those with similar blogs or sites and exchange links
4. They help you connect to folks with similar likes
5. It tells you that users are engaged on your site/ blog
How do search engines look at comments to value your blog?
1. Search engines take comments as an indication of your blog’s likeability
2. They count it as a vote for your blog post
It’s not equal to a link but certainly has value