As a general rule, the more comments a blog post has, the more authoritative it appears to be. And that does good things for your brand and overall traffic numbers.
The BackType Wordpress plugin has the potential to take commenting to a whole new level on your blog. In short, BackType will pull the conversation about an individual blog post from multiple sources on the Web and publish them in the post's comments section. You set up the sites from which you want to import comments and BackType does the rest. Current supported sites include Twitter, FriendFeed, Digg, Reddit, Hacker News, and comments from other blogs that link to your post.
Ultimately, the BackType plugin's value will be determined by relevence. If comments are not 100% on topic, you stand the chance of having an irrelevant comment section, and even the appearance of spam.
However, if it works correctly, there could be some major benefits. Immediately, you are getting fresh content on a continuous basis. And that means the search engines will likely index your page more often. The post also stands to generate even more insightful comments from regular site visitors, in a kind of watershed effect. More comments also means more page views. Users want to view the conversation sparked by their comments, and will come back to see follow up comments and to make more comments. It also increases your page's time-on-site; the more comments, the more people read. Finally, a widely-read post with multiple comments has a way of getting linked to from other sources - SEO gold, and possibly starting another round of commenting, so the cycle starts all over again.