Forcing people like me to join a forum or social site is just so hard to do, if not impossible. And even worse when I see people restricting their blog comments only to people who are registered with the blog.
As a rule of thumb, you should never restrict readers from leaving a comment on your site. The job is to make things easier for people to come to you, not building another wall so high people just leave.
If you’re afraid of spam, get antispam plugins, modules or whatever that can get that done.