I see an advantage of including the facebook commenting system. The advantage that I see is that the comments are centralised and that given both being social networking platforms with a wider reach, there's a possibility of greater outreach. Like when someone comments on your blog using facebook, everyone associated with your fb profile/page will get a notification about a new comment. The other advantage I can see is suppression of chances of Spam/Anonymous comments when using these platforms, as someone having an account only can comment using these plugins, other's can't.
However, beyond the (supposed) disadvantage you outlined, there's another huge disadvantage associated with using these custom commenting systems on blogs. Like for example, I'm having an account with both, Facebook as well as Disqus, and I prefer not to post comments using my FB profile, and I've somehow forgot the password for my Disqus account, thus leaving me no option to comment on a blog, when I want.
I took 20 minutes to post just 10 lines 