I understand the need to lock people into the boss room, but locking them out who haven't entered yet? That's a pain.

That said, I've seen several times where lockouts occur - not necessarily resulting in wipes - because classes who can speed through race to the lockout barrier. Not just Ninjas, with their running speed, but I see a lot of other classes attempting to get there first. They wait 5 seconds or so, then dash in, assuming anyone still running will make it into the barrier in time.