There are advantages for bloggers who install this system. Firstly, because Facebook is so hugely popular, prospective commenters are often already logged into the site. So they don't have to fill in the comment form. It's just a little thing but it makes commenting a bit more likely.
Then there's the fact that it reduces spam, and perhaps abusive comments as well. Because most people on FB use their real identities and these are what they'll be using when commenting, they are more aware of how they present themselves.
They're worth keeping an eye out for if you are a blogger wishing to comment as well. If you are logged in as your Facebook Page not only will your comment appear in the thread of that particular blog post but it will also appear as an update on your Page as well, along with a link to that blog post and an image from it (if there is one for Faceook to grab). So you're effectively getting "two for the price of one".