This debate is held in NH as a runup to the NH primary. I doubt that all the "joke" or "vanity" candidates are on the NH primary ballot. Hunter certainly is, and is doggedly campaigning there, God bless him. Go Hunter!
Correction to myself: a quick search found one site (I couldn't find the official site) which says there are 21 Rep. candidates on the NH primary ballot, so lots of true fringe types must be on there. However, I still doubt than many of them have campaigned there, given interviews on local media, or even visited the place. Hunter has. Paul has lots of support. The "front runners" who will be in the debate will spend all their time arguing over who flip-flopped the most, anyway. Who wants to hear that again? I'd almost rather watch Ron Paul screeching maniacally and doing his Queeg impersonation.