It is more or less like buying articles for your blog. Some people do it really well. These are mostly bought when a forum is new or the admin couldn't generate enough activity on their forum. Hiring someone will look like there is enough discussion going on and that is good for the forum.
@Abhi I much prefer my own content, and if it takes a while to generate real traffic, then so be it. You can't honesty claim you've succeeded with a blog if the posts, comments, and traffic are all fake.
Phenomlab for example is three years in the making and only now starting to climb the ranks. I've finally reached the stage where I'm happy with the functions, content, and layout.