@Vladimir Prelovac -

@Handy Rev: Actually the reason is practical and that is fighting spam. Sites like these are exposed to thousands of spammers and we want our content to stay clean and curated. Banning a Facebook or Twitter account is much more effective than banning an email address (which is produced much more quickly).

The Facebook/Twitter requirement is exactly the reason I don't comment on WPMU.org blog posts. Sorry. I'm not connecting my Facebook account to anything (because Facebook can't be trusted with my privacy), and I don't connect my Twitter account to anything I don't control.