As far as people minding what they say, do you really believe it's impossible to create a fake FB account, or that it's even that hard to do? Besides, I know some of the commenters here, and they're not the kind of people that would be deterred from expressing their opinions if you knew who they were IRL.
Facebook would certainly make things easier for us, but I don't think that just using the comment plug-in would allow the integration and features we want.
Facebook's also a huge pain to work with, because - much like how they change it for users constantly - they change it for developers, too, meaning any work you put into Facebook integration has to be re-done every year or so. Unfortunately, that means we don't have the resources to create - and maintain - login via Facebook.
We do have upgrades planned for the discussion system, though. It will at least be easier and faster to use.