It's a topic as old as the blog it's self. Should you let people put comments on your blog or not? there is really no 100% right or wrong answer. What it comes down to is are they helpful to you?
To start with there are two ways to treat comments. You can allow anyone to comment, and trust your spam filter to catch the really bad ones. OR, you can set it to where you have to moderate (check out) every comment that is left.
The first way is risky. Search engines do block some sites, and having a link to one of those sites even if only in your comments can lower your cred with the search engines. Plus spam filters wont always be able to catch all of the trash. So you will still occasionally have to take a look over what people have been leaving.
The second way isn't too big a deal when the site is getting started, but can become a problem if you start having to deal with dozens of comments every day. Many you can just approve, other you might have to edit, or delete. Then there is the fact that most people that leave comments like to see you answer them.
So let us look at the important stuff.
1. Do comments help you build traffic? No, not really. The vast majority of people that visit your site wont leave any. You should have a forum, and if they want to talk with you, they can do it there. Plus if someone really wants to say something about your post/article and they do it on a comment, it's more likely they wont say anything on their own blogs/sites/profiles. Since backlinks or trackbacks are a good thing, you are hurting yourself by using comments.
2. Are the social aspects of comments worth the headache for you? Again, not really. If you start getting dozens of comments a day that does mean you are getting lots of traffic, but it also means you are spending a lot of time on something that could be better spent. Writing new material, and promoting your site for example.
So comments are not important for blogs. They tend to be more of a headache than they are worth, and they don't do anything to help your site a forum can't do better. I think a single link back from someone else's site is worth a hundred comments.
Lastly, and this is where the blogging for money part comes in... they can be a deterrent to your income. As long as you are honest about what you are doing, they can't do too much harm, but you can't please everyone all the time. There is nothing worse than having a ticked off customer storming around your site leaving comment after comment about how you scammed them, or some such non-sense.
No matter how hard you try, you will always run into people that think their failure to use the product you sold them, means it has to be your fault. Allowing comments gives them a way to get back at you (really themselves) in a potentially harmful way to your business.