Let’s talk about traffic:
Many times someone will write about your topic on their blog. You can then go there and leave a comment and (if a conversation breaks out) perhaps a link. Plus it’s a good way to find business partners, because you’ll be able to find the obscure sites you didn’t previously know existed.
Another type of “blog comment” is question answers. You can go to wikianswers.com or Yahoo! Answers and answer questions people are asking about your niche. Answer a few in your niche, and people will start going to your profile and clicking on your links. (You can also use these questions and answers as a FAQ page on your site).
But don’t answer with one or two answers. Write! Be the expert and give expert answers. Wow people so they have no choice but to ask “how is this phenom?”
Clearly the main benefit will be traffic. . . but a side benefit that can not be undersold is the relationships you can create with people in your niche that you never would have found. Think you’ll ever need people to sell your bundle product? Promote your webinar? Buy your book?
Oh. . .how could I forget? What about the links? We talk about incoming links and link juice all the time. The idea that you could get get 365 incoming links in one year from all kinds of sites related to your site, that is incredible.
There just isn’t a lot of downsides to blog commenting. So get on it.