Blog comments help your blog in many ways, one, you get fresh content, two, comments show how popular your blog is, and three, it reduces your bounce rate (because the commentator spends some time writing comments. However, getting comments is really difficult. Even when people like your post, they might not be interested in leaving comments. Buying blog comments can help.
Not just keeping comments flowing but trying to keep new blogs with new content feeding. I have not posted a blog on my end for months ;( As most go straight to the Forums and then read a story about a pig XD
I had paid people to comment in the passed, not like it's a big deal but I did not see any benefits from it.
Buying blog comments is a really good way to keep your blog active but it really depends on how much you have to be able to invest into the blog for comments if you choose to pay for them. The only other issue when buying blog comments would be you as the owner keeping fresh content flowing so that people can make the comments that you are paying for.
Looking into blog comment exchanges as well is always a great way to get blog comments but also help someone else out in the process and is perfect if you do not have a lot of money to invest into paying for blog comments.