No.
1. Fucks up the the keyword density. I guess you can adjust the page/post after every comment but that seems like it would be a hassle.
3. Comments detract from the message. Distracting noise.
2. Most people are idiots. I don't want idiots loitering on my websites. Enjoy, buy, leave, come back, repeat.
3. Without comments, surfers have three options. They can either buy something, leave the website, or go to a different page on the website. Turning on comments gives them an additional option.
4. The act of leaving a comment makes people feel as if they've accomplished something when, at least as far as I'm concerned, they've accomplished nothing.
5. I'm of the belief that the people who tend to leave comments are less likely to be people who are willing to pay for something
With all that being said, there are most certainly benefits to allowing comments and just because commenters are likely to be nothing more than virtual loiterers, they can still be of use from a time spent/pages viewed perspective and the increased rate of frequency that pages/posts are updated helps with SEO but I think those benefits can only be actualized if you actively attempt to make commenting a larger part of the user experience and actively encourage surfers to leave comments. If you plan to just turn on comments and hope that you get a few it's not worth it.
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